Homebrew: Underwater Mechanics and 3D combat in Roll20 (D&D 5e)

"There is heroism and brute warfare on the ocean floor, unseen by land-dwellers. There are gods and catastrophes."
~ China Mieville, The Scar

That which lies below the waves is an interesting and terrible place indeed. It is also a great location for stories and campaigns, and as such saddens me that I cannot enjoy it more often. One of the primary reasons for this is due to the additional rulings, which are seen as limitations on characters that want to flex their entire skillset. Personally I am of the belief that these rules should not be seen as limitations, but as demonstrations of the pressures on aquatic life, and as a way to explore how these pressures have been overcome or coexisted alongside.

Below are some rulings that can be used by DMs that want to emphasise the effect of being in an aquatic environment on characters and objects. These rules would also be good to consider for subterranean games, as many underground rivers and sea caves and the like exist below the earth.

Core Rules

Official rules related to underwater gameplay can be found in the following areas:
Player's Handbook: breathing underwater/suffocation (pg 183); underwater combat (pg 198)
Dungeonmaster's Guide: extreme cold, frigid water, high altitude aka effects of pressure (pg 110); swimming (pg 116); underwater visibility (pg 117)
Ghosts of Saltmarsh: ocean environs (pg 202-207); currents, depth illumination (pg 203); pressure and objects (pg 204)

In addition, Appendix A of Ghosts of Saltmarsh includes rules on ships and naval combat; Storm King's Thunder also includes a ruling on damage at 3,000 feet below sea level (pg 203).

Wildshape

Druid Wildshape can now swap aquatic and terrestrial ordering of learnt forms. At 2nd level, you can turn into creatures with a swim speed only, and at 4th level you can begin to learn forms with a walking speed. Forms available at 2nd level would therefore include the quipper (or any other Tiny fish), dolphin, and seahorse. At the DM's discretion, 2nd level druids may also have access to forms that have both a walking and swimming speed (octopus, frogs, snakes, crabs, and their giant variants).

Talking

Sound travels further in water than on land, but communication for humanoids becomes harder when underwater. Languages designed to be spoken underwater - such as Aquan or Sahuagin - can be spoken normally, but languages such as Common become warped and take more time to be clearly conveyed. Terrestrial languages cannot be whispered underwater, and shouting is not much more audible than 'normal' speech volume.

Due to the inability to whisper, DMs may wish to add sign language as an additional language to be learnt, or as a dialect to their connected languages. This requires the communicating creatures to be able to see each other, as well as for the one talking to have one or more hands free.

Carrying Capacity

A player character can only carry up to half their terrestrial carrying capacity whilst underwater. The average diver is advised to carry a maximum of 15 lbs on their person, so I feel that an average Strength PC being able to carry 75 lbs maximum is more than reasonable.

Lightning Damage

These rules specifically apply to lightning caused by spellcasting. Lightning from storms rarely breaches far below the waves, quickly dissipating into the water. Arcane lightning however reacts somewhat differently.

When a spell that only affects one target is used that deals lightning damage, lightning leaves the space of the target and travels through the water. The lightning disperses over a circular radius equal to 5 feet for every level of the spell (minimum of 5 feet). Creatures within this radius - including the caster - must succeed on a Constitution saving throw against the caster's spell save DC. On a failed save, the creature takes damage equal to the damage dealt to the initial target (if the attack missed, roll damage as if you had hit). On a successful save, the creature takes half damage.

If the spell has an area of effect (such as lightning bolt) that can hit multiple targets, a radius effect spreads from each affected target, as described above.

Heavy Armour

A character cannot swim in land-made heavy armour. Heavy armour made specifically for underwater wearing can be found and used. If you wear heavy armour and do not meet the prerequisite Strength requirements for wearing heavy armour, you begin to sink at a rate of 5 feet per round.

Conditions

When a creature is knocked prone, they are instead disorientated and rotated in the water, and must use half their movement to reorientate themselves.

Creatures that are paralysed, stunned, or otherwise physically incapable of moving to keep themselves stable remains where they are unless a water current or other external force moves them.

Spells

These conditions occur when a spell is cast underwater.

• Reverse gravity does not move water, as the spell specifies objects and creatures are moved. Damage is taken in open water if creatures are moved the entire 100 feet, due to the decreasing pressure occurring over a short period of time.
• Stinking cloud and other such abilities that release poison, spores, clouds etc. in the area dissipate and dilute in 1d4 rounds.
• Gust of wind and other such spells produce water currents.
• Wall of water and watery sphere produce the same effects as usual, isolating regions of water in order to create the spell effects. These areas of water can be identified with a successful Intelligence (Investigation) check against the spellcaster's DC; otherwise they appear as regular parts of the water.
• Spells that create audial effects - such as thunder spells and illusions - can be heard from double their terrestrial range.

3D Combat in Roll20

"It has been given many names. Each inlet and bay and stream has been classified as if it were discrete. But it is one thing, where borders are absurd. It fills the spaces between stones and sand, curling around coastlines and filling trenches between the continents."
~ China Mieville, The Scar

Aquatic combat is not a 2D space. Combatants are not all on the same plane, the same height patch of land. They dive up and down, ambush from below, wriggle through streams and ambush from the murky depths surrounding them. This is a fairly simple but imperfect system that can be used to help players get a sense of depth.

The map texture below comes from 2-Minute Tabletop.

To start with, we have our underwater scene and a single character - the merfolk. When starting a map scene, the group determines a depth below sea level to be treated as the X-axis of our 3D grid. As the merfolk is the only player on the field, her current depth - let's say 25 feet below sea level - is considered the 0 of our X-axis.

Suddenly, an ambush! At this point the shark and two sahuagin enter the fray, coming in different directions but also at different depths. Depth is represented using the coloured tokens that can be accessed in the token status effect section:

Blue represents Below, as in below the X-axis. Red represents above. Each 0-9 value on the colour - made by clicking the colour and then immediately pressing a the desired number - represents the number of squares (units of 5 feet) below the X-axis the creature is. In the above example, the shark and the merfolk are both at the same depth. Meanwhile, Sahuagin 1 is 15 feet below the merfolk and Sahuagin 2 is 20 feet below the merfolk, putting them at 40 and 45 feet below sea level.

In this way, the group has a rough guide of distances between one another. Whether or not you then go ahead to figure out hypotenuses and the like is up to the group; as rule of thumb, if you're treating diagonal movement as 5 feet, you could probably do the same for taking depth into consideration.

One round later and the scene has changed: Sahuagin 1 has moved up by 15 feet, removing their colour icon as they are now on the X-axis. Sahuagin 2 and the merfolk have moved upwards, now at 5 feet below sea level. The giant shark meanwhile has sunk down, changing from a red icon to a blue one.

Vertical movement underwater is treated the same as regular movement for creatures with a swimming speed. Creatures without a swimming speed treat moving downwards as difficult terrain (i.e. as regular swimming would be for them), but treat swimming upwards as normal movement speed (this ruling is a WIP, need to figure out exactly how I want to do this). Creatures that are over-encumbered find the reverse; moving down is easy, but it's a lot harder to swim back up again.

If you would like to see more of my underwater homebrew, please consider checking out my selkie and medusoid races.