The Sea, Underwater Environments, and Waterborne Vehicles

Swimming

Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you're swimming. You ignore this extra cost if you have a swimming speed and use it to swim. At the DM's option, gaining any distance in rough water might require a successful Strength (Athletics) check.
Unless aided by magic, a character can't swim for a full 8 hours per day. After each hour of swimming, a character must succeed on a DC 10 Constitution saving throw or gain one level of exhaustion.
A creature that has a swimming speed—including a character with a ring of swimming or similar magic—can swim all day without penalty and uses the normal forced march rules above.
Swimming through deep water is similar to traveling at high altitudes, because of the water's pressure and cold temperature. For a creature without a swimming speed, each hour spent swimming at a depth greater than 100 feet counts as 2 hours for the purpose of determining exhaustion. Swimming for an hour at a depth greater than 200 feet counts as 4 hours.

Suffocating

A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).
When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.
For example, a creature with a Constitution of 14 can hold its breath for 3 minutes. If it starts suffocating, it has 2 rounds to reach air before it drops to 0 hit points.

Underwater Combat

When adventurers pursue sahuagin back to their undersea homes, fight off sharks in an ancient shipwreck, or find themselves in a flooded dungeon room, they must fight in a challenging environment. Underwater the following rules apply.
When making a melee weapon attack, a creature that doesn't have a swimming speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is a dagger, javelin, shortsword, spear, or trident.
A ranged weapon attack automatically misses a target beyond the weapon's normal range. Even against a target within normal range, the attack roll has disadvantage unless the weapon is a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).
Creatures and objects that are fully immersed in water have resistance to fire damage.

The Sea

Characters can row a boat for 8 hours per day, or can row longer at the risk of exhaustion (as per the rules for a forced march in Adventuring ). A fully crewed sailing vessel can sail all day, assuming its sailors work in shifts.

Navigation

Seagoing vessels stay close to shore when they can, because navigation is easier when landmarks are visible. As long as a ship is within sight of land, there is no chance of the vessel becoming lost. Otherwise, a ship's navigator must rely on dead reckoning (tracking the direction and distance of the ship's travel) or the sun and the stars.

Visibility Outdoors

When traveling outdoors, characters can see about 2 miles in any direction on a clear day, or until the point where trees, hills, or other obstructions block their view. Rain normally cuts maximum visibility down to 1 mile, and fog can cut it down to between 100 and 300 feet. On a clear day, the characters can see 40 miles if they are atop a mountain or a tall hill, or are otherwise able to look down on the area around them from a height.

Visibility at Sea

A relatively calm sea offers great visibility. From a crow's nest, a lookout can spot another ship or a coastline up to 10 miles away, assuming clear skies. Overcast skies reduce that distance by half. Rain and fog reduce visibility just as they do on land.

Underwater Visibility

Visibility underwater depends on water clarity and the available light. Unless the characters have light sources, use the Underwater Encounter Distance table to determine the distance at which characters underwater become aware of a possible encounter.

Underwater Encounter Distance

Creature Size	Encounter Distance
Clear water, bright light	60 ft.
Clear water, dim light	30 ft.
Murky water or no light	10 ft.

Weather

d20	Temperature
1-14	normal for the season
15-17	1d4 × 10 degrees Fahrenheit colder than normal
18-20	1d4 × 10 degrees Fahrenheit hotter than normal

d20	Wind
1-12	no wind
13-17	light wind
18-20	strong wind

d20	Precipitation
1-12	no precipitation
13-17	light rain or light snowfall
18-20	heavy rain or heavy snowfall

Extreme Cold

Whenever the temperature is at or below 0 degrees Fahrenheit, a creature exposed to the cold must succeed on a DC 10 Constitution saving throw at the end of each hour or gain one level of exhaustion. Creatures with resistance or immunity to cold damage automatically succeed on the saving throw, as do creatures wearing cold weather gear (thick coats, gloves, and the like) and creatures naturally adapted to cold climates.

Extreme Heat

When the temperature is at or above 100 degrees Fahrenheit, a creature exposed to the heat and without access to drinkable water must succeed on a Constitution saving throw at the end of each hour or gain one level of exhaustion. The DC is 5 for the first hour and increases by 1 for each additional hour.
Creatures wearing medium or heavy armor, or who are clad in heavy clothing, have disadvantage on the saving throw. Creatures with resistance or immunity to fire damage automatically succeed on the saving throw, as do creatures naturally adapted to hot climates.

Strong Wind

A strong wind imposes disadvantage on ranged weapon attack rolls and Wisdom (Perception) checks that rely on hearing. A strong wind also extinguishes open flames, disperses fog, and makes flying by nonmagical means nearly impossible. A flying creature in a strong wind must land at the end of its turn or fall.
A strong wind in a desert can create a sandstorm that imposes disadvantage on Wisdom (Perception) checks that rely on sight.

Heavy Precipitation

Everything within an area of heavy rain or heavy snowfall is lightly obscured, and creatures in the area have disadvantage on Wisdom (Perception) checks that rely on sight. Heavy rain also extinguishes open flames and imposes disadvantage on Wisdom (Perception) checks that rely on hearing.

Owning a Ship

At some point during the campaign, the adventurers might gain custody of a ship. They might purchase or capture one or receive one to carry out a mission.

Waterborne Vehicles

Ship	Cost	Speed	Crew	Passengers	Cargo (tons)	AC	HP	Damage Threshold
Galley	30,000 gp	4 mph	80	—	150	15	500	20
Keelboat	3,000 gp	1 mph	1	6	1/2	15	100	10
Longship	10,000 gp	3 mph	40	150	10	15	300	15
Rowboat	50 gp	1½ mph	1	3	—	11	50	—
Sailing ship	10,000 gp	2 mph	20	20	100	15	300	15
Warship	25,000 gp	2½ mph	60	60	200	15	500	20

Crew. A ship needs a crew of skilled hirelings to function. As per the Player's Handbook, one skilled hireling costs at least 2 gp per day. The minimum number of skilled hirelings needed to crew a ship depends on the type of vessel, as shown in the Airborne and Waterborne Vehicles table. You can track the loyalty of individual crew members or the crew as a whole using the optional loyalty rules below. If at least half the crew becomes disloyal during a voyage, the crew turns hostile and stages a mutiny. If the ship is berthed, disloyal crew members leave the ship and never return.
Passengers. The table indicates the number of Small and Medium passengers the ship can accommodate. Accommodations consist of shared hammocks in tight quarters. A ship outfitted with private accommodations can carry one-fifth as many passengers.
A passenger is usually expected to pay 5 sp per day for a hammock, but prices can vary from ship to ship. A small private cabin usually costs 2 gp per day.
Cargo. The table indicates the maximum tonnage each kind of ship can carry.
Damage Threshold. A ship has immunity to all damage unless it takes an amount of damage equal to or greater than its damage threshold, in which case it takes damage as normal. Any damage that fails to meet or exceed the damage threshold is considered superficial and doesn't reduce the ship's hit points.
Ship Repair. Repairs to a damaged ship can be made while the vessel is berthed. Repairing 1 hit point of damage requires 1 day and costs 20 gp for materials and labor.

Vehicle Proficiency

If you have proficiency with a certain kind of vehicle (land or water), you can add your proficiency bonus to any check you make to control that kind of vehicle in difficult circumstances.

Rowed Vessels

Keelboats and rowboats are used on lakes and rivers. If going downstream, add the speed of the current (typically 3 miles per hour) to the speed of the vehicle. These vehicles can't be rowed against any significant current, but they can be pulled upstream by draft animals on the shores. A rowboat weighs 100 pounds, in case adventurers carry it over land.

Officers and Crew

Each vessel requires people to run it: the officers in charge and the sailors who follow their orders.

Types of Officers

If you'd like to explore running a ship, it needs officers to oversee its operations—officers who fill six different roles. Some roles aboard a ship reflect the need for trained experts to direct a crew's efforts. Other roles focus on keeping the crew's health and morale in order. The roles are meant to provide a sense of the types of ability checks useful to managing a ship. Of these, though, captain is the only role that must be filled for the ship to function. A ship needs a single person to issue orders and respond to threats, otherwise a ship risks chaos and confusion during a crisis.
Each type of officer is described below, along with the abilities and proficiencies that help a character excel in that role:
Captain. The captain issues orders. The best captains have high Intelligence and Charisma scores, as well as proficiency with water vehicles and the Intimidation and Persuasion skills.
First Mate. This specialist keeps the crew's morale high by providing supervision, encouragement, and discipline. A first mate benefits from a high Charisma score, as well as proficiency with the Intimidation and Persuasion skills.
Bosun. The bosun (or boatswain) provides technical advice to the captain and crew and leads repair and maintenance efforts. A good bosun has a high Strength score, as well as proficiency with carpenter's tools and the Athletics skill.
Quartermaster. The quartermaster plots the ship's course, relying on knowledge of nautical charts and a study of weather and sea conditions. A reliable quartermaster tends to have a high Wisdom score, as well as proficiency with navigator's tools and the Nature skill.
Surgeon. The ship's surgeon tends to injuries, keeps illnesses from spreading throughout the ship, and oversees sanitation. A capable surgeon benefits from a high Intelligence score, as well as proficiency with herbalism kits and the Medicine skill.
Cook. A ship's cook works with the limited ingredients aboard a ship to make meals. A skilled cook keeps the crew's morale in top shape, while a poor one drags down the entire crew's performance. A talented cook has a high Constitution score, as well as proficiency with brewer's supplies and cook's utensils.

Crew Members

A ship requires a number of able-bodied sailors to crew it, as specified in its stat block. A crew's skill, experience, morale, and health are defined by its loyalty score. It decreases as a crew takes casualties, suffers hardship, or endures poor health. It increases if the crew enjoys high morale, has good health care, and receives fair leadership.
A typical crew member uses the commoner stat block in the Monster Manual.

Loyalty

Loyalty is an optional rule the DM can use to determine how far an NPC party member will go to protect or assist the other members of the party (even those he or she doesn't particularly like). An NPC party member who is abused or ignored is likely to abandon or betray the party, whereas an NPC who owes a life debt to the characters or shares their goals might fight to the death for them. Loyalty can be role played or represented by this rule.

Loyalty Score

An NPC's loyalty is measured on a numerical scale from 0 to 20. The NPC's maximum loyalty score is equal to the highest Charisma score among all adventurers in the party, and its starting loyalty score is half that number. If the highest Charisma score changes—perhaps a character dies or leaves the group—adjust the NPC's loyalty score accordingly.

Tracking Loyalty

The DM can choose to keep track of an NPC's loyalty score in secret so that the players won't know for sure whether an NPC party member is loyal or disloyal (even if the NPC is currently under a player's control).
An NPC's loyalty score increases by 1d4 if other party members help the NPC achieve a goal tied to its bond. Likewise, an NPC's loyalty score increases by 1d4 if the NPC is treated particularly well (for example, given a magic weapon as a gift) or rescued by another party member. An NPC's loyalty score can never be raised above its maximum.
When other party members act in a manner that runs counter to the NPC's alignment or bond, reduce the NPC's loyalty score by 1d4. Reduce the NPC's loyalty score by 2d4 if the character is abused, misled, or endangered by other party members for purely selfish reasons.
An NPC whose loyalty score drops to 0 is no longer loyal to the party and might part ways with them. A loyalty score can never drop below 0.
An NPC with a loyalty score of 10 or higher risks life and limb to help fellow party members. If the NPC's loyalty score is between 1 and 10, its loyalty is tenuous. An NPC whose loyalty drops to 0 no longer acts in the party's best interests. The disloyal NPC either leaves the party (attacking characters who attempt to intervene) or works in secret to bring about the party's downfall.

Mutiny

A poorly led or mistreated crew might turn against its officers. If the crew's loyalty reaches 0, the crew mutinies. They become hostile to the officers and might attempt to kill them, imprison them, or throw them overboard. The crew can be cowed into obedience through violence, combat, or offers of treasure or other rewards.
When the DM ends the mutiny, the crew's loyalty score increases by 1d4.

Siege Weapons

Ballista

Large object
Armor Class: 15
Hit Points: 50
Damage Immunities: poison, psychic
Cost: 125 gp
A ballista is a massive crossbow that fires heavy bolts. Before it can be fired, it must be loaded and aimed. It takes one action to load the weapon, one action to aim it, and one action to fire it. Available on a Galley (4), Keelboat, Sailing ship, and Warship (2).
Bolt. Ranged Weapon Attack: +6 to hit, range 120/480 ft., one target. Hit: 16 (3d10) piercing damage.

Cannon

Large object
Armor Class: 19
Hit Points: 75
Damage Immunities: poison, psychic
Cost: 2,000 gp
A cannon uses gunpowder to propel heavy balls of cast iron through the air at destructive speeds. In a campaign without gunpowder, a cannon might be an arcane device built by clever gnomes or wizardly engineers. A cannon is usually supported in a wooden frame with wheels. Before it can be fired, the cannon must be loaded and aimed. It takes one action to load the weapon, one action to aim it, and one action to fire it. A canon can replace a mangonel on a Galley (2) or Warship (2).
Cannon Ball. Ranged Weapon Attack: +6 to hit, range 600/2,400 ft., one target. Hit: 44 (8d10) bludgeoning damage.

Mangonel

Large object
Armor Class: 15
Hit Points: 100
Damage Immunities: poison, psychic
Cost: 250 gp
A mangonel is a type of catapult that hurls heavy projectiles in a high arc. This payload can hit targets behind cover. Before the mangonel can be fired, it must be loaded and aimed. It takes two actions to load the weapon, two actions to aim it, and one action to fire it. A mangonel typically hurls a heavy stone, although it can hurl other kinds of projectiles, with different effects. Available on a Galley (2), Sailing ship, and Warship (2).
Mangonel Stone. Ranged Weapon Attack: +5 to hit, range 200/800 ft. (can't hit targets within 60 feet of it), one target. Hit: 27 (5d10) bludgeoning damage.

Naval Ram

Large object
Armor Class: 15
Hit Points: 100
Damage Immunities: poison, psychic
Cost: 500 gp
A naval ram is guaranteed to hit regardless of the results of the attack roll, but if the attack roll is successful then the attack is automatically a critical hit. Available on a Galley and Warship.
Ram. Melee Weapon Attack: +8 to hit, reach 5 ft., one object. Hit: 16 (3d10) bludgeoning damage.

Trebuchet

Huge object
Armor Class: 15
Hit Points: 150
Damage Immunities: poison, psychic
Cost: 1,000 gp
A trebuchet is a powerful catapult that throws its payload in a high arc, so it can hit targets behind cover. Before the trebuchet can be fired, it must be loaded and aimed. It takes two actions to load the weapon, two actions to aim it, and one action to fire it. A trebuchet typically hurls a heavy stone. However, it can launch other kinds of projectiles, such as barrels of oil or sewage, with different effects. A canon can replace two mangonels on a Galley or Warship.
Trebuchet Stone. Ranged Weapon Attack: +5 to hit, range 300/1,200 ft. (can't hit targets within 60 feet of it), one target. Hit: 44 (8d10) bludgeoning damage.