PaulS:
If so, I'm sure that they would have been suggested by now. That none have implies that there aren't. And, if you think about it, the sensors would have to be able to communicate with each other, meaning that both need to be powered. You want to swim around with a battery strapped to your wrist
Just kicking around ideas—I'm open to anything powered or not. Kind of assumed it'd have to be powered. I haven't delved too deep into the Arduino community and assumed there had to be some sort of pair of sensors that could tell when they were within a few feet of each other. My thought is that if I can get something that works, I can whittle it into a more aerodynamic design afterwards.