Sort of like assigning the ability of speech a random number between 1 n 5or10 (speech number) then when you try to speak UW a random number between 1 n 5or10 is selected, if number = speech number then a successful conversation underwater, if not -X% of hp (maybe 5%)?
The other idea i had was drink a potion n it forms a bubble around u, where the squares are no longer /UW, the area effect being like light and depending on whether it's lesser/norm/greater. Lesser=area like mod 4 light area, norm=area like mod 40light and greater= area like mod 130light. This allows people to enter the bubble and also use it for speaking. Only problem with potion is it could be abused but the time on it could be around 2mins.