Attack rolls underwater are made at disadvantage, unless it makes sense for a character to fight easily underwater.

For any creatures that can’t breathe underwater, if you want to create tension around how long they can hold their breath, you can create a Breath Countdown die. Set the starting value at 3 or more, and tick down as follows:

• Tick down once whenever any PC takes an action underwater.
• If an action roll is a failure or with Fear, you can use your move to tick it down an additional time. (If the roll is both a failure and with Fear, you can tick it twice instead.)

Once the countdown ends, whenever any PC takes an action, the underwater PC must mark Stress. ( See “Countdown” for more on Countdown Dice.)