unity standard asset problem: water disappears

Hello,

I‘ve created just normal water in a terrain with the Environment package from the Unity Standard Assets. But after clicking the Play-Button the water disappeared. It didn‘t just disappeared from the szene view, also from the hierarchy. And I absolutly don‘t know why. Google didn‘t helped. I use the unity student version and I am a beginner without any experience. Maybe the following Information helps for better understanding of the issue: I noticed that the water which I draged and droped into my terrain, had in the hierarchy the word „(clone)“ behind the name. And it wasn‘t possible to rename it.

I hope anyone can help me. I would be very thankful.