Attack rolls underwater have disadvantage unless it makes sense for a character to easily fight underwater, like a Siren or Shark making an attack on a PC.

For any creatures that can’t breathe underwater, you can use a countdown to create tension about how long they can hold their breath. Set the countdown die’s starting value to 3 or higher 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, a PC underwater must mark a Stress whenever they take an action.