ACR Survival Res-Q Whistle
Here's what other sites are saying:
Strap the ACR WW-3 Survival Whistle to your pack or PFD, and you'll be heard in an emergency. The WW-3's flat, bright orange design is lightweight and visible in low light, so there's no reason you shouldn't have it close at hand. Dual chambers flush water and debris easily, and the shrill dual tone can be heard over rushing rapids, high wind, and engine noise. Currently in use with NASA and the U. S. Navy, the ACR WW-3 Survival Whistle is USCG and SOLAS approved.