Another option to consider is the flat ACR whistle. We have them attached to all our life jackets on the boat, and there's one on my keychain, and my son's backpack. They are light, thin, and require less air than some of the other pea-less whistles. Here's a review from Backpacking Light:

http://www.backpackinglight.com/cgi-bin/backpackinglight/acr_emergency_whistle.html

They are about $3 each. I bought them at West Marine. They are also USCG approved.