Someone mentioned that your feet touch the pool wall when you turn. If this is true, then could you hang a flat sensor board over the pool edge? A pressure activated switch could be used to increment a counter and a display (in a waterproof container) could be embedded in the board to display the laps underwater. A simple non-retriggerable one-shot multivibrator could be used to de-bounce the switch closure.
This is effective if someone had a personal lane. The touch pads are already in existence for races and events, though most are bulky.
Much of the time—in my case at least—lanes are split in half when I'm at a public pool. Which is why I was seeking a personal, portable solution that could clip onto a suit or goggle, or some sort of band that could be worn.
Okay, I see.
Water and air have similar magnetic permeabilities so perhaps a very strong magnet could be worn on your wrist or ankle and sensed by a coil of wire near the pool end wall. Suitable de-bouncing (seconds) would prevent multiple triggers during the turn.