Underwater Combat (5e Variant Rule)
Underwater Combat
The standard 5e rules for underwater activity and combat seem to lack the tension and flavor that adventures in this environment deserve. I developed these homebrew rules from various sources, google searches on holding your breath, a bit of math, and playtesting. These rules should give a great roleplaying experience for almost all water situations, such as traversing underwater tunnel, fight with creature trying to drag you under, falling into the water while in armor, etc.
Movement
Unless you have a swim speed, swimming is treated like difficult terrain effectively halving movement. Your underwater swimming speed is the same as your surface swimming speed. Your armor and encumbrance will have an uncontrolled affect on your swim speed and direction. In addition to the intended, spent, movement that the character does, the character will also move straight down at 5 feet per round if holding the equivalent of light armor, or 15 feet per round if holding the equivalent of medium armor, or 25 feet per round if holding the equivalent of heavy armor. An athletics (swim) check of DC equal to the character’s AC (without magic bonuses, but with disadvantage if armor gives the stealth disadvantage) will half this sinking movement. This will likely require the character to spend some, if not all, their movement to counter the downward sinking movement, or speed their descent if this is their desired movement direction.
Combat
Melee attacks are made at disadvantage underwater, unless using dagger, javelin, spear, or trident.
Ranged attacks are made at disadvantage underwater, unless throwing any of the weapons listed in Melee attacks above or while using a crossbow. Ranged weapons cannot go past their “normal” range bracket.
Armor
If you are in water with your armor, likely you will want to take it off. The don/doff table for PHB 146 lists doff time (converted to rounds). This could be halved if you have assistance. You can attempt to remove your armor in the water, but you may not use your move during this time and it counts as doing your action (using up “breath” if submerged). If you have something sharp, you may attempt to cut your armor off with a successful DC 15 Dexterity (Acrobatics) check to cut the doff time in half.
| Category | Don | Doff |
|---|---|---|
| Light Armor | 10 rounds | 10 rounds |
| Medium Armor | 50 rounds | 10 rounds |
| Heavy Armor | 100 rounds | 50 rounds |
| Cloths | 5 rounds | 5 rounds |
| Backpack | 1 round | 1 round |
| Shield | 1 action | 1 action |
Holding Your Breath
Each character has a number of rounds of “breath” equal to 5xCON+20. If you spend 10 consecutive rounds breathing deeply (as your action), you may add 10 rounds of breath. While underwater, certain activities will expend extra rounds of “breath” from your reserve:
- Taking an action expends ½ round of “breath”
- Doing a move to swim expends ½ round of “breath”
- Moving at 5’ swim rate expends no extra rounds of “breath”
- If you take damage while underwater, you must make a CON based concentration check or loose 1 round of “breath”
Once you run out of “breath”, you must make a CON save vs:
DC = 10 + #rounds out of breath + damage taken/10 + 1/move + 1/action Fail: lose ½ total max HP (or auto death save if already dead) and disadvantage on next CON save (due to taking in water)
It takes an action to take a breath, which restores 5 rounds of “breath”
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