Deep Sea Diving

Underwater stories are a bit glossed over in the official Player's Handbook and Dungeon Master's Guide, and this falls in line with the general philosophy of simplifying the game mechanics to promote a more narrative and less number-crunchy story. However, for those of us interested in more elaborate ocean adventures, here are some options to challenge players seeking riches far beneath the waves. Like all homebrew, the values and ranges presented here are more like guidelines, and are completely open for your particular table to change and interpret for what suits your game best.

Below the Depths

Whether you're playing in a fantasy world or a more science accurate one, just having the ability to breathe underwater is not enough once you reach a certain point below sea level. The incredible vastness of the ocean creates several dangerous conditions for creatures to deal with, affecting humanoid biology in very particular ways. Every level of depth comes with its own complications, and those risks accumulate the deeper one travels.

100 feet down

Once a humanoid dives below 100 feet, the symptoms of Nitrogen Narcosis begin to manifest. Also known as "raptures of the deep," affected creatures begin acting as if drunk, often with impaired judgment and losing the ability to think critically. When spending more than 1 minute at this depth, a creature must make a DC 10 Constitution saving throw for every minute spent at this level or lower. On a failure the diver gains the poisoned condition until they spend at least 1 minute above this depth. Like all the risks described here, these symptoms will continue to progress as depth increases.

200 feet down

Past 200 feet, the affects of narcosis increase and amplify. This is compounded with the symptoms of Oxygen Toxicity, in which the very air we need to survive slowly begins to destroy living tissue. Every minute, creatures below this depth must make a DC 12 Constitution saving throw, taking 2d6 poison damage on a failed save, or half as much damage on a successful one. Every 10 minutes spent at this depth increases a creature's exhaustion level by one.

300 feet down

This is the maximum safe diving depth for humanoids, and continuing further increases chances of death exponentially. Euphoria and hallucinations become common, and the senses of sight and sound can become overwhelmingly sensitive. In addition to the previous conditions, creatures below this depth must also make a DC 15 Constitution saving throw every minute, taking a further 4d6 poison damage on a failed save, or half as much damage on a successful one. Every 10 minutes spent at this depth increases a creature's exhaustion level by three.

400 feet down

This is the general depth of the Continental Shelf, a mostly flat sea floor that extends about 500 feet from the coast. This is the foundation for most of what we know about ocean ecosystems, home to the coral reefs and all the life that surrounds them. Sunlight still reaches these depths, fueling the plants and plankton that feed everything else. In a fantasy world, this is the most likely home for tritons, merfolk, and other aquatic humanoids.

600 feet down

If you go to the edge of the continental shelf, you reach the steep drop-off known as the Continental Margin. This is like a sheer cliff that descends towards the true ocean floor. Diving down to 600 feet, you will reach what is sometimes called the disphotic zone, where light from the surface is significantly dimmed and plants can no longer grow. This is a region of dim light, and sunlight is not strong enough here to affect those with sunlight sensitivity. Every minute, creatures below this depth must make a DC 12 Constitution saving throw, taking 2d6 cold damage on a failed save, or half as much damage on a successful one.

2,000 feet down

At this depth, the pressure of the ocean begins to take its toll on humanoid bodies. Every minute, creatures below this depth must make a DC 12 Constitution saving throw, taking 2d6 bludgeoning damage on a failed save, or half as much damage on a successful one.

3,000 feet down

This is the maximum depth that sunlight can be seen. Below this is a region of complete darkness. In addition to the previous conditions, creatures below this depth must also make a DC 15 Constitution saving throw every minute, taking a further 2d6 bludgeoning damage and 2d6 cold damage on a failed save, or half as much damage on a successful one.

The Ocean Floor

At an astounding 15,000 feet below the surface and 900 feet from shore, we reach the Abyssal Plain, the vast stretch of emptiness that connects the continents. Almost nothing can live at these extreme depths, and those few creature that do are often strange and horrifying. While sunlight cannot reach this far, life finds a way. Some creatures survive off of detritus and waste matter that drifts down from the surface regions, others thrive using the chemicals spewed from large cracks and vents in the bedrock. If you want to include this region in your game, it should be appropriately dangerous and alien.

Decompression Sickness

Often called the bends or taravana, this occurs when ascending too quickly through the ocean and can have deadly consequences. Symptoms include joint and limb pain, dizziness, and difficulty breathing. Extreme cases can even result in seizures, paralysis, and death. At a depth below 100 feet, a creature ascending faster than 30 feet in one minute must make a DC 12 Constitution saving throw every minute, taking 2d6 poison damage on a failed save, or half as much damage on a successful one. This damage cannot be reduced from poison resistance or immunities.

Deep Sea Diving Gear

Below are some options for both technological and magical items used to survive the ocean depths. A full tank of air should grant an experienced diver approximately 2 hours worth of air, although consumption will increase with depth.

Diving Equipment

Name	Depth Rating	Weight	Properties
SCUBA System, with tank	--	50 lbs	--
SCUBA tank (air)	200 ft.	30 lbs	--
SCUBA tank (compressed air)	400 ft.	30 lbs	Advantage on saves against oxygen toxicity
Atmospheric Diving Suit	2,000 ft.	900 lbs	Negates the bends, ocean pressure, nitrogen narcosis, and oxygen toxicity
Experimental Liquid Breathing System	400 ft.	50 lbs	Negates the bends, nitrogen narcosis, and oxygen toxicity

Magic Item Name	Depth Rating	Weight	Properties
Potion of the Deep	400 ft.	--	Water breathing, negates nitrogen narcosis and oxygen toxicity
Potion of the Abyss	15,000 ft.	--	Water breathing, negates ocean pressure, nitrogen narcosis, and oxygen toxicity
Helm of the Deep	400 ft.	5 lbs	Requires attunement; water breathing, negates nitrogen narcosis and oxygen toxicity