Some suggestions from my Beginner Triathlete board:

1) Use the letters of the alphabet for each 50. Go up to J = 500.

2) After each 50, state what you are on outload, underwater. "100", "150", etc.

3) Buy a counter that goes on your finger (they have those).

4) Use your lap counter on your watch.

5) Use pennies on the side of the pool. Make a pile beforehand with how many laps you need to complete (count a 50 as a lap). After each 50 reach up and move the pennies to the completed pile.