I fit right in with swamp, typically they are cheap, so i toss one in about everywhere. Recently, i started adding to all my packs as well. I use ITW Nexus (mfg) brand-they fit most sternum straps on all my packs-its give you a whistle built in to the buckle http://www.itwnexus.com/catalog/index.php/dw/op/a/8/c/24/p/76

PS if you play it cool and dont drill them with requests, you can order"samples" aka a neat way to get a free one.

...and as one mentioned "except when swimming" i recently started adding one to my wrist on a little plastic springy wrist loop thing.......needed to signal the lifeguard, whom was "adjusting his umbrella (not saying it was his fault by no means " when i seen a small child almost drown at disney last year.