Many cats perceive their feeding area as a major part of their territory. Therefore, your cat probably considers her water bowl as a safe place to keep her prey.
Your cat probably just enjoys playing with water and watching it move around. So, if your cat puts things in her water bowl – she might just like dropping things in water!
Our cat sees her water bowl as the main part of her nest, and she might be moving fresh prey to this safe location – in order to teach you how to hunt! This is a part of cats’ natural instincts.
This might be just a practical thing for your cat – she can’t drink water while holding a toy in her mouth, so, she will drop anything she has in her mouth to the bowl to be able to drink water.