how to you signal to your buddy underwater?

[ballastbelly]

...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

[baghira]

Does not apply to freediving mate..
The only thing is the continusous attention of your buddy..

[EmicMalta]

if scuba, tehre are horns that can be connected to the second stage system, but are not so effectivly

[rule3]

nothing better than a middle finger.... haha

[CARP3R]

Shout....ha ha

[Granitu]

high frequency whistle is the best