PeterH:
It's tempting to suggest using RFID, but that wouldn't work through water. Could you arrange to carry an RFID tag in such a way that it was out of the water when you passed the sensor? I guess the sensor would be at the end of the lane, where you are turning.
Yeah, I figured RFID was off the table. I'm doing flip turns, so body parts are briefly out of the water, but nothing that would be close or accurate enough to read RFID from what I've seen.
Are there any two sensors/radios/etc that could talk/read/connect to each other when within that 3–4 foot proximity? (with one or both underwater)