Some means of signaling help is part of the ten essentials of camping and whistles are an inexpensive, effective choice. What’s the best Scout whistle? The Storm Whistle is the loudest whistle in the world; 75% louder than your average whistle. I haven’t tested the claim that it can be heard fifty feet under water, but I believe it.
I used one of these to signal a canoes that were separated from our group a half a mile or more away and out of sight. They heard the signal clearly.