Aquatica
Rules For Underwater
Adventures
These rules are meant to make underwater combat more fun, without making it easier or harder. Some mechanics have been added to provide more tactical options. Others have been changed from all-or-nothing outcomes into more dramatic struggles with many shades of success or failure.
If you find vertical positioning to be tedious to track on a traditional top-down battle map, try switching to a side-view map while underwater!
Movement
While swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a swimming speed.
The Dash action cannot be taken underwater by a creature that lacks a swimming speed.
Breath
A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).
When a creature ends their turn breathless and underwater they must lose 1 hit die or fall unconscious. When an unconscious creature starts their turn underwater, they lose 10 health.
A creature immediately loses their breath if they are critically hit or if they fall into water unexpectedly.
Communication
Most languages cannot be spoken underwater. Aquan can be readily spoken and understood, but otherwise the only means of communicating underwater is with rudimentary sign language.
Casting
When a creature casts a spell that requires verbal components, they exhale their remaining air. After they finish casting, they become breathless.
Pressure
The bodies of most humanoids were not built to withstand the pressure of deep sea diving. For every 100 feet a character dives, they gain a level of Exhaustion. Characters with a swimming speed are immune to diving sickness.
Buoyancy
At the start of their turn a creature will sink unless they spend movement staying in place. Sinking does not provoke Attacks of Opportunity. The distance a creature sinks depends on their buoyancy, which is based on the armor they are wearing. Values are in feet, with negative representing sinking, and positive representing floating.
| Armor | Buoyancy |
|---|---|
| Heavy | -20 |
| Medium or Light | -10 |
| None or Natural | 0 |
Creatures with a swimming speed are not affected by buoyancy unless they wish to be. Creatures who are climbing or otherwise holding onto something stationary are not affected by buoyancy. Creatures who are grappling or being grappled sink or float half the normal distance, and drag their partner along.
As an action, characters may attempt to cut their armor off. Doing so requires a blade and a successful DC15 Dexterity (Sleight of Hand) check. This action can also be taken for a willing or unconscious ally within 5 feet.
Attacking
Attacks with weapons that deal bludgeoning or slashing damage are made with disadvantage.
Ranged weapon attacks automatically miss against targets more than 30 feet away.
Damage Types
Spells which deal Fire or Thunder damage behave somewhat differently underwater.
Fire Damage is halved while underwater, after which resistance and vulnerability apply as normal.
Underwater spaces consumed by fire become Heavily Obscured for one round, due to the cloud of bubbles caused by the heated water. Fire spells which affect a single creature consume only that creature's space.
Thunder Damage is deafening underwater. Any creature that takes 10 or more thunder damage from a single source is also deafened for one minute.
Lightning Damage spreads underwater. When a creature takes lightning damage, all other creatures within 10 feet take 1d6 lightning damage. This does not affect creatures who already took damage from the triggering effect.
Feats
Deep Diver
You have trained to explore the deepest recesses of the ocean and live to tell the tale.
- Increase your Strength or Constitution by 1.
- You can hold your breath for 5 extra minutes and you do not lose your breath from falling into water unexpectedly.
- You have advantage on Sleight of Hand checks made to cut off armor.
- You are immune to the effects of water pressure.
Expert Swimmer
You have spent years pushing your body to its limits in the water, and as a result your aquatic athleticism is unparalleled.
- You gain a swimming speed of 30 feet, unless you already have a faster swimming speed.
- You have advantage on Athletics and Acrobatics checks made while in the water.
Items
Diving Armor
Uncommon Armor
This armor is covered in the skin of some unknown aquatic creature. A slime-covered bladder dangles limply from the neck of the armor, and three tabs hang from key points on the chest.
The wearer can insert the bladder into their mouth to gain the ability to breathe underwater. The bladder provides 10 minutes of air per day, refilling at dawn.
As a bonus action, the wearer can pull a tab to achieve one of the following effects:
- Their buoyancy becomes 0
- Their buoyancy becomes +20 (causing them to rise to the surface)
- Their buoyancy returns to its normal value
Sea-Elven Weapon
Common Weapon
This weapon continuously breathes a small amount of air from its surface, leaving a fine trail of bubbles when held underwater. Attacks made with this weapon are not affected by the normal penalties from being underwater.
Bubble Kelp
Common Wondrous Item
This bit of dried kelp is often found tucked into the belts of experienced sailors. It is covered in tiny hardened bubbles which, when eaten, refill the eater's lungs with breath.
Aboleth Saliva
Uncommon Wondrous Item
This sticky substance is a milder version of an Aboleth's terrifying mucus. When rubbed onto a creature's gums, that creature gains immunity to the effects of water pressure for 24 hours.
Babeltad
Common Wondrous Item
This tiny tadpole can fit easily within a person's mouth, and will happily live there for as long as it is allowed. A person hosting a babeltad finds themselves able to speak clearly underwater. The babeltad does not grant them the ability to speak or understand Aquan, it merely allows their normal languages to carry through water as if through air.
Babelfish
Uncommon Wondrous Item
After a few weeks, a babeltad will mature into a babelfish. Like its younger siblings, a babelfish will happily reside inside a creature's mouth, cleaning their teeth. In return, it allows its host to speak clearly underwater, regardless of what language they are speaking. While the host is speaking underwater, the babelfish also translates their words into Aquan, allowing its host to speak (but not understand) the language of the oceans.
Image Credits:
Pierre Denys de Montfort's Poulpe Colossal attacks a merchant ship, 1810
A. Pollock's Octopus verrucosus, 1886
