Whistle Fox 40

It is supposed to be indestructible. I cannot attest to that, but I have carried mine around for a decade and it still works and is still loud. It is so loud that I do not let my son play with it. They also make a glow-in-the-dark version. It works when wet but $6 for a plastic whistle seems a little steep but not if your life depends on it. It can be heard from a quite a distance. I have signaled friends hundreds of feet away in the woods and they definitely heard it.