Ashbridge'ss Bay Park: This is one of the places I've had a number of water fights in.
Place has a nice mix of terrains from the sandy beach to the grassy picnic grounds and various amounts of trees/bushes in areas.
This area farther out on the peninsula is typically a little less busy that the beach area, but offers decent amount of space to hold small-to-mid-sized battles.
There are public washrooms in the area with showers with clean water, but there's also the lake for those less picky. There are also various water fountains, but their flow rates tend to be quite slow.