Divers don't talk that much, actually.

See, there is an air intake in your mouth - and trying to talk under water does not exactly result in the noises you expect when you're talking into air.

The sign used by divers to alert others is banging your knife against the air tank. There are actual hand signs for 'I'm in trouble' or 'I'm out of air'.

Fair enough, like I say I'm not familiar with the subject but curious. I was more picturing the masks that look a bit like gas masks that have radios in them. My extent of diving knowledge is limited to movies and Mythbusters, I'll admit.

Thanks for that information :)

Sign language is the answer.