Short wave at 800 metres depth.

[colly0410]

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..

[turretslug]

"Director's Licence" methinks! Conceivably some sort of modulated carrier acoustic link?- but not very secure and short-range.

[Russell W. Barnes]

Once you're beneath the waves, you talk to no-one. You listen, but you say nowt. That's the idea of submarines!

[Michael Watterson]

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.