Compressed air powered, a combination of underwater and above water signalling device, just connect it to your low-pressure inflator hose and the BCD connection. 

A simple push of the button gives out a 115-125dB sound underwater.  Above water, the horn produces a whistle of 125-130dB!  Select the suitable connection to fit your BCD connector (if you're not sure, just ask).

The product should be thoroughly cleaned in fresh water after diving to ensure performance.

Warning: Keep away from close proximity to ears when using, or personal injury may occur.