Do water trampolines actually work like regular trampolines?

Yes, water trampolines do work like regular trampolines. However, they lose substantial bounce due to being on an inflated island on the water. You can get a visual of how much bounce you get out of one here:

Water bouncers have even less bounce. However, both water trampolines and water bouncers are extremely fun. Especially if you add accessories to them such as a blast bag. You'll be on the water for hours!