Regarding propellor cavitation... I can hear DD's pass overhead from within the sub all too well.
When listening to the Hydrophone... you can also hear the DC splashes.
One thing that might be nice is to maybe make new Ogg files for the voices... with distant splashes followed by "Wasserbomben!"
(as long as they aren't all the same voice type... There are nine voice types in SH3 ranging from Mickey Mouse to The Incredible Hulk.)