Water boatmen eat things that shouldn't be in your pool: algae, tiny bugs or small amphibians.

There are 3 reasons why there's something in your pool to eat:
1. You have algae. If so, eradicate it.
2. You have bugs that are living in your pool . . . but such bugs live on algae. See #1
3. You have bugs that are landing on your pool, because of night lights OR the reflections of night lights. Turn off the lights; block the reflections.
4. You have bugs that are randomly migrating or falling into your pool. This is not easy to fix; use the solutions above.