I use the Fox 40 and take it with me on every dive. I attach it to my left chest d-ring using a double ended bolt snap. Not sure if it works underwater. Never thought to try. I have it just in case I have an emergency on the surface and need to alert others. Heck of alot better than shouting. I typically dive quarries or freshwater lakes and there are always other divers around. The sound of a whistle is universally known as a signal for help, so I'm sure I'd get other's attention pretty quickly.