Two things:
-Get a scarecrow (fake owl, etc.) of some sort to scare them away.
-Add a rock that protrudes above the water so the birds can get out of the water if they fall in. Birds are terrible swimmers (excluding ducks, geese, swans, and other water birds), so raising the water level might not make that big of a difference since the water would weigh the bird down a good bit. Trust me on the swimming thing; my grandmother has lost multiple chickens to drowning since they fell into a duck pond full to the top with water (or my favorite, one managed to fall into a water bucket twice as tall as it is and drown). The rock would allow the bird to at least get out of the water to dry out before flying away.
This should take care of the problem for at least the most part.