Just been watching a film on 'Movies for Men' (Freeview ch 48) called 'Sun Attack'. There were two submarines at 800 metres depth talking to each other using Short Wave radio! & here was me thing that they'd have to use 'Very Low Frequency' or even 'Extra Low Frequency' radios at that depth. Oh how I laughed..
Indeed stealth is the point of subs. Radio (can be aerial between towed buoys) is seriously unstealthy. Hydrophone (direct audio or ultrasonic sound) is not a good idea either.