Is there any sort of setup where two small sensors would be able to identify their proximity to each other? (or, if they're simply within a certain proximity in general)
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?
Now, you could use a ping type sensor at the end of the lane, to determine when you, or anyone else, is close. They come in a variety of cone angles, to limit how far off center an object can be before it triggers the sensor.
The problem that I see is that anyone approaching the sensor would trigger it, which may make it impractical.