Simple Underwater Rules

By Sarah Lynn

Suppose you're going on an underwater adventure with potions of water breathing, or a spell that makes you sprout gills, or whatever. Here are some simple rules.

Movement

There are two options:

Going Light: You have a tendency to float upwards. If you do nothing, you will rise towards the surface. You will move by swimming. If you are exploring an underwater dungeon, you will essentially be flying through the dungeon. Your movement is ½ if you have both hands free, otherwise it is ¼. (Note: You cannot wear any armor heavier than studded leather and take this option.)

Going Heavy: You weight yourself down and sink right to the bottom. You will move by walking along the seabed. If you are exploring an underwater dungeon, you will do so in the normal manner and may carry weapons or gear as normal. Your movement is ¼.

Magic

Players can cast spells as usual. Spells may function differently underwater, however.


Combat 

Slashing and bludgeoning type weapons do 1/2 damage. Piercing weapons do full damage. Normal missile weapons don't work underwater, except for crossbows. Underwater, their range is halved. At a distance of 20' or greater, they do 1/2 damage.

Drowning

If you are not surprised, you have can hold your breath for your constitution divided by 3 rounded down in minutes. If you are surprised you can do half of that. Once your time is up you make a saving throw. If you fail the role then you die.  If you make the roll, then you have not drowned. Congratulations, you have 1 more minute to live! When that's up roll again each minute with a cumulative -4 penalty.

Equipment

Everyone must write down on a sheet what they're carrying and where. Scrolls and spell books will be destroyed underwater. Potions had better have a tight cork or wax seal. (Drinking them underwater is difficult too: 50% chance 1/2 of doses are lost in the process.) Since they cannot light torches or lanterns, characters who cannot see in the dark will need a magical light source if they enter a dark place.

Mapping

You can't map underwater. You may draw a crude map without measurements, on the theory that your characters will have a better handle on their environment than they will (especially session to session). But that's it. 

Communication

Characters probably do not have a way of talking to one another underwater. They must communicate through gestures. No one can hear you scream underwater. For the same reason, all underwater creatures receive a +1 on surprise rolls against PCs.