I'm having trouble with underwater sounds as well. I'm currently using a blowfish in a underwater mission, and all my gals are using nude "armor" with the exception of one, who is using the basic panty-only underwater "armor".
In version 2017-04-24-4, it says "Sound 212 in BATTLE.CAT not found" as soon as I try to move a nude gal inside the craft.
In versions 2017-05-10 and 15, it says it's sound 7212 instead.
Strangely, the gal dressed differently can move at will. Sadly, that wasn't enough to win the mission, so I had to abort.