It looks like things like fitbit can track swim lengths.
You might be able to use an accelerometer to get a crude estimate of position or direction. Integrating acceleration gives velocity and integrating velocity gives you position. That its basically adding values. It's usually noisy data and the value drifts a lot but it might be good enough to tell which end of a pool you are at.
You could use haptic feedback (e.g. vibration motor) to tell you how far through a pre-set distance you are so you don't gave to look at a display.
You could wear an armband or swim cap with a colour code (single colour or combination)
Something like this
http://www.cmucam.org/projects/cmucam5/ ... olor_Codes
When the colour code is detected you are in view on another lap.
You could do something similar with QR codes or the like.
You could wear an IR LED that sends an IR remote control code and a receiver poolside. Setup so that it only reads the transmitted code at short range. Each period of a few seconds where reads occur is a lap.
You might be able to limit the range of a 433 MH transmitter / receiver pair, BLE, ZigBee etc so that it only connects at short range (say <5m) at one point on a lap by using directional and/or low gain antennas.
RFID is probably too short range, especially if it has to get through water.