Aquatic Terrain

Aquatic terrain is the least hospitable to most adventurers because most can’t breathe there. Aquatic terrain doesn’t offer the variety that land terrain does. The ocean floor holds many marvels, including undersea analogues of any of the terrain elements described elsewhere, but if characters find themselves in the water because they were bull rushed off the deck of a pirate ship, the tall kelp beds hundreds of feet below them don’t matter. Accordingly, these rules simply divide aquatic terrain into two categories: flowing water (such as streams and rivers) and non-flowing water (such as lakes and oceans).
Flowing Water: Large, placid rivers move at only a few miles per hour, so they function as still water for most purposes. But some rivers and streams are swifter; anything floating in them moves downstream at a speed of 10 to 40 feet per round. The fastest rapids send swimmers bobbing downstream at 60 to 90 feet per round. Fast rivers are always at least rough water (Swim DC 15), and whitewater rapids are stormy water (Swim DC 20). If a character is in moving water, they move downstream the indicated distance at the end of their turn. A character trying to maintain their position relative to the riverbank can spend some or all of their turn swimming upstream.
Swept Away: Characters swept away by a river moving 60 feet per round or faster must make DC 20 Swim checks every round to avoid going under. If a character gets a check result of 5 or more over the minimum necessary, they arrest their motion by catching a rock, tree limb, or bottom snag—they are no longer being carried along by the flow of the water. Escaping the rapids by reaching the bank requires three DC 20 Swim checks in a row. Characters arrested by a rock, limb, or snag can’t escape under their own power unless they strike out into the water and attempt to swim their way clear. Other characters can rescue them as if they were trapped in quicksand (described in Swamp Terrain).
Non-Flowing Water: Lakes and oceans simply require a swim speed or successful Swim checks to move through (DC 10 in calm water, DC 15 in rough water, DC 20 in stormy water). Characters need a way to breathe if they’re underwater; failing that, they risk drowning. When underwater, characters can move in any direction.
Stealth and Detection Underwater: How far you can see underwater depends on the water’s clarity. As a guideline, creatures can see 4d8 × 10 feet if the water is clear, and 1d8 × 10 feet if it’s murky. Moving water is always murky, unless it’s in a particularly large, slow-moving river.
It’s hard to find Cover or Concealment to hide underwater (except along the sea floor).
Invisibility: An invisible creature displaces water and leaves a visible, body-shaped “bubble” where the water was displaced. The creature still has Concealment (20% miss chance), but not total Concealment (50% miss chance).

Underwater Combat

Land-based creatures can have considerable difficulty when fighting in water. Water affects a creature’s attack rolls, damage, and Movement. In some cases a creature’s opponents might get a bonus on attacks. The effects are summarized below. They apply whenever a character is swimming, walking in chest-deep water, or walking along the bottom of a body of water.

Underwater Combat Modifiers

Condition	Slashing/Bashing Attack	Slashing/Bashing Damage	Piercing Attack	Piercing Damage	Movement	Off Balance
Freedom of Movement	normal	normal	normal	normal	normal	No
Has a Swim speed	–2	half	normal	normal	normal	No
Successful Swim check	–2	half	normal	normal	quarter	No
Firm footing	–2	half	normal	normal	half	No
None of the above	–2	half	–2	half	normal	Yes

• Creatures flailing about in the water (usually because they failed their Swim checks) have a hard time fighting effectively. An off-balance creature loses its Dexterity bonus to Armor Class, and opponents gain a +2 bonus on attacks against it.
• A creature without Freedom of Movement effects or a Swim speed makes Grapple checks underwater at a –2 penalty, but deals damage normally when grappling.
• Creatures have firm footing when walking along the bottom, braced against a ship’s hull, or the like. A creature can only walk along the bottom if it wears or carries enough gear to weigh itself down: at least 16 pounds for Medium creatures, twice that for each size category larger than Medium, and half that for each size category smaller than Medium.

Ranged Attacks Underwater: Thrown weapons are ineffective underwater, even when launched from land. Attacks with other ranged weapons take a –2 penalty on attack rolls for every 5 feet of water they pass through, in addition to the normal penalties for range.
Attacks from Land: Characters swimming, floating, or treading water on the surface, or wading in water at least chest deep, have improved Cover (+8 bonus to AC, +4 bonus on Reflex saves) from opponents on land. Land-bound opponents who have Freedom of Movement effects ignore this Cover when making melee attacks against targets in the water. A completely submerged creature has total Cover against opponents on land unless those opponents have Freedom of Movement effects. Magical effects are unaffected except for those that require attack rolls (which are treated like any other effects) and fire effects.
Fire: Nonmagical fire (including Alchemist's Fire) does not burn underwater. Spells or spell-like effects with the fire descriptor are ineffective underwater unless the caster makes a Caster Level check (DC 20 + spell level). If the check succeeds, the spell creates a bubble of steam instead of its usual fiery effect, but otherwise the spell works as described. A supernatural fire effect is ineffective underwater unless its description states otherwise. The surface of a body of water blocks line of effect for any fire spell. If the caster has made the Caster Level check to make the fire spell usable underwater, the surface still blocks the spell’s line of effect.
Spellcasting Underwater: Casting spells while submerged can be difficult for those who cannot breathe underwater. A creature that cannot breathe water must make a Concentration check (DC 15 + spell level) to cast a spell underwater (this is in addition to the Caster Level check to successfully cast a fire spell underwater). Creatures that can breathe water are unaffected and can cast spells normally. Some spells might function differently underwater, subject to GM discretion.

Underwater Hazards

The Bends

“The Bends” happens when gas bubbles appear in the bloodstream from rapid depressurizing. While within the range of depth tolerance, a creature does not have to worry about this effect; their body is well accustomed to changes in pressure in that range. However, if a creature becomes acclimated to a pressure that is much different than he is used to, and quickly ascends to shallower depths, this change in pressure can cause terrible effects. Plants, Constructs, and Undead are immune to the bends.
100 feet per minute is the maximum rate of ascension to avoid any ill effects. A character can rise 100 feet in one round without ill effects, so long as they don’t raise another 100 feet for at least a minute. A character that rises more than 100 feet in a minute will take 1d4 Constitution damage per each additional 100 feet traveled in that minute.

Water Currents

Water currents move at a particular speed in a particular direction. They can add to your speed if you wish to go in the same direction of the current, otherwise they subtract from your base speed. If the current is faster than a character’s base speed, they are swept away at a rate equal to the speed of the current minus any of their base speed that they wish to sacrifice to the current, in the same way that this is done for buoyancy. For example, if a character enters a water current of speed 100 feet and their base speed is 60, they will be swept away for at least 40 feet per round for as long as they are in the current. Water currents that run vertically can affect buoyancy.
Currents have 3 statistics; speed, direction, and diameter. The speed is the speed at which a character is swept away while in it. The direction is the direction in which the character is swept. The diameter is how wide and tall the current is, which could be anywhere from 5 feet to several miles.
Currents often change direction like a winding river.
Water currents can be both a hazard and a convenient means of transportation. Most are easily detectable by sound, sight, and touch. Often they are warmer or colder than their surroundings, depending on their point of origin. Generally, larger currents are slower moving and go very long distances, while smaller ones move quickly for short distances.

Riptide

A rip current, or riptide, is a strong channel of water flowing seaward from near the shore, typically through the surf line. Usual flow is at least 10 feet, and can be as fast as 50 feet. They can move to different locations on a beach break, up to a few hundred feet a day. They can occur at any beach with breaking waves. Rip currents are stronger when the surf is rough (such as during high onshore winds, or when a strong hurricane is far offshore) or when the tide is low.
A riptide can have a diameter of 10 to 50 feet, moves often but does not quickly dissipate, and always has a direction opposite the shoreline. The best course of action when caught in a riptide is to swim perpendicular to the current, thus eventually leaving it.

Undertow

Undertows are below surface rushes of water returning to sea after coming ashore as breaking waves. They typically pull swimmers away from shore and into breaking waves that can submerge even strong swimmers. If there is an area under the waves, such as a break in a sandbar, where water can flow back out to sea more easily, a narrow rip current can form. If there is no weak point in the surf, then the water flows back out to sea under the waves, forming an undertow.
An undertow is a temporary current with a speed of 5 to 25 feet and a diameter of 5 to 20 feet with a downward and seaward direction. Unlike a riptide, they typically dissipate in 1d4 rounds.

Whirlpools

A whirlpool is a swirling body of water typically formed by ocean tides. Most whirlpools are not very powerful, nor particularly long lived. More potent ones are more appropriately termed maelstroms, and can be permanent fixtures of the sea. While very powerful whirlpools are created in narrow shallow straits with fast flowing water, the most powerful maelstrom are formed near portals to the Elemental Planes of Water.
Whirlpools come in one of four sizes: vortex (10 to 50 feet in diameter), standard whirlpool (51 to 100 feet in diameter), maelstrom (101 to 500 feet in diameter), and greater maelstrom (501 feet to 2,000 feet in diameter). Whirlpools are typically as deep as they are wide.

Whirlpool Statistics

Whirlpool Type	Range Increment	Rounds Trapped	Damage per Round
Vortex	25 feet	1d4	1d4
Standard	120 feet	1d8	1d8
Maelstrom	300 feet	2d10	1d10
Greater	1000 feet	4d12	2d6

Whirlpools are surrounded by strong feeder currents that can draw in swimmers far from the whirlpool itself. These currents work like strong water currents that always move towards the center of the whirlpool. The speed of a feeder current immediately next to a maelstrom is 120 feet. This speed is divided in half at each range increment from the center of the maelstrom. Table: Whirlpool Statistics describes the length of each range increment based on the size of the whirlpool. The current is generally negligible if its speed is less than five feet.
For example, a feeder current that exists 950 feet from the center of a maelstrom moves at a speed of 15 feet, while a greater maelstrom’s feeder currents at the same distance would still have a speed of 120 feet.
Once a swimmer is sucked into the whirlpool by the feeder currents rushing toward it, the victim becomes hopelessly trapped. During each round of being trapped the victim takes bludgeoning damage from being violently whipped around and battered.
Table: Whirlpool Statistics indicates the number of rounds trapped and the bludgeoning damage for each round that the victim is trapped, based on the size of the whirlpool.
A vortex can only trap objects or creatures of Huge size or smaller. Gargantuan-sized creatures or greater cannot be trapped by standard whirlpools.
Maelstroms and greater maelstroms can trap creatures or objects of any size.
At the end of being trapped, the victim is propelled at a speed of 120 feet through the bottom of the whirlpool. This could be the equivalent of being violently shoved to the sea floor, squeezed through a hole in the bottom of the ocean, or punted into the Elemental Plane of Water — depending on what is at the bottom of this terrible peril.