...if you need to get his attention urgently? scuba divers have a big tank they can clang with their knife, but what would you have handy to make a sound? thanks
Does not apply to freediving mate..
The only thing is the continusous attention of your buddy..
if scuba, tehre are horns that can be connected to the second stage system, but are not so effectivly
nothing better than a middle finger.... ;) haha
Shout....ha ha
high frequency whistle is the best ;)