The ocean makes an interesting adventuring environment because it is so inherently deadly. Characters can drown, freeze to death or die from the enormous, crushing pressure from water above. These dangers should never be far from players minds as they take their characters beneath the waves. For you as a DM, this gives you plenty to work with to craft challenging encounters and instill fear or a sense of alienation in your players.

Almost all characters will rely on magic effects of some kind or another to preserve them while underwater (druids are the best off here because often they can get away with just wild shape). A truly fiendish DM will subject the characters to some kind of dispel magic effect. This is a low blow, and an enemy caster hitting the party with an area dispel will likely catch them off-guard. Suddenly you’ve silenced the player spellcasters (and bards), killed verbal communication between the characters, and set a timer for death by drowning.

Possible effects for a suddenly unprotected party trapped in the briny deep:

  • Drowning. Without the ability to breath water, the PCs are living on borrowed time as measured by the Drowning Rule
  • Inability to cast spells. Again, without the ability to breath water most spellcasters will be shut down. Sorcerers with Silent Spell and preparation casters who have Silenced spells prepared are the exception.
  • Inability to verbally communicate. It’s possible the party has set up a telepathic link or other sort of non-verbal communication. When their water breathing is dispelled, most parties will wish they had! As DM you should decide to what degree to enforce the barrier to communication. I would allow big gestures and noises (like a warning shout) to communicate a few very basic concepts, and require uses of Bluff / Perform and Sense Motive for complex directions.
  • Environmental dangers: cold and pressure. Very cold water (like that found more than a few tens of feet below the surface of the ocean) deals 1d6 points of nonlethal damage per minute. For every hundred feet below the surface, players are subjected to 1d6 nonlethal damage from crushing pressures. This can be negated with a DC 15 Fort save once per minute (+1 for every previous check).
  • No light Depths under fifty feet tend to be low-light conditions, and one hundred to two hundred feet should be pitch black (depending on weather, time of day, clarity of water, etc.) You should of course eyeball the appropriate depth depending on the encounter you wish to create. Underwater the characters’ primary means of illumination is likely magical. If it is dispelled the players will find themselves blind in a cold alien world.

These are a lot of really nasty effects and will likely throw the party off-balance. Any challenging fight coupled with a successful dispel of some sort becomes a fight to remember as in the blink of an eye an underwater adventure becomes an underwater nightmare.

Keep in mind you shouldn’t do this if the players have no escape. If five hundred feet beneath the surface the players lose their water breathing and the only way to regain it is with a verbal spell, the situation will end in Total Party Kill and no one will be happy (if you’re a kind DM you’ll play with action points and the spellcaster can spend one for Silent Spell).

Depressurization dangers are real as well. In 1983 the Byford Dolphin accident occurred, where 4 men were killed when subjected to explosive decompression. While in a decompression chamber at 9 atmospheres the hatch was suddenly released. From the article about the diver who suffered the worst effects: “All of his thoracic and abdominal organs, and even his thoracic spine were ejected, as were all of his limbs.”. This reality is truly shocking and goes beyond what is depicted in film. This leads me to wonder where I could use it in a darker campaign.

Beyond explosive decompression, the bends is a danger for sudden ascents. If any character quickly surfaces, you might want to try this rule:

The Bends
For every hundred feet ascended in under 10 minutes, the character must make a Fortitude save DC 20 + 1 for every hundred feet. Failure subjects the character to 1d6 Constitution damage for every hundred feet ascended as the character suffers wracking pain and internal damage from the bends.

This rule may seem a little too “scientific”, though, so only incorporate it if it fits the flavor of the campaign.

For deep adventures, remember that there are varying degrees of depth. Not all aquatic monsters are comfortable at great depths, and not all horrors of the deep will pursue characters into shallower depths. Keep this in mind and you can give a more realistic feel to the oceanic realm and give yourself more design space:

  • Surface waters. Bright, warm and almost friendly, the surface waters conjure up images of rays of sunlight slanting across bright coral reefs, sparkling merfolk cities and shallow, shark infested wrecks.
  • Ocean bottom (moderate depth). Here the light is dim, the water cold (still significantly above freezing), and non adapted creatures may suffer a bit from pressure effects (100-300 feet down). The sahuagin and kuo-toa live down in these murky depths, plotting against their mortal enemies. Deep wrecks are slowly patrolled by giant squid.
  • Black depths. Most aquatic creatures cannot venture this deep without suffering from the cold and pressure. Only specialized monsters lurk in these unknowable fathoms. The pressure is enormous (5-15d6 / minute) and the waters are freezing. It’s never lighter than pitch black, and some regions of water may become so oxygen depleted even aquatic creatures suffocate.

For inspiration for monsters, you can look to the real world. There are plenty of evocative creatures in our own oceans. I’ll briefly highlight relatively recent documentation of our own mysterious giant, Architeuthis:

YouTube Video of Live Giant Squid: