Actually, there is a sensor that sits up under the closed flaps (shouldn't be exposed) that activates once water surrounds it. NG's would scare me. I haven't seen one do that. Most usually have to be fully submerged before they activate. Mine are Crewsaver models from the UK. Mine is top of the line models. Luckily, I got them free. LOL. But work for my calm water. But, just got the vest of my dreams. It'll be my new lifevest. It's a military tactical lifevest (not inflatable). Has lots of pockets. Just did an edit, here's an actual picture of my vest. It's actually a flat black, camera flash makes it look more brownish green with black highlights.

Not as bulky as it looks, and can row in it (used by quite a few Elite forces). My buddy in the Navy had one, and I fell in love with it. Just found one of my own (and ALOT cheaper then the $500 price tag on them lol). Nicest thing of all, one size fits all, good bouyancy, is also can be used as a fishing vest, plus can add a hydration bladder in the back (is designed for one actually). Cost me less then buying a William Joseph Coastal Pack plus is an approved floatation device (these are now being issued to the Coast Guard and some Navy units). So got the coast guard approval there too. LOL