Unity Water4 looks like it can pass through objects from a distance

Hello guys, so I was creating my island terrain and i used water4 for the ocean, then when i added trees, it looked like the water passes through the trees when you look at it from far away.
Here’s a screenshot:

I’m pretty new to designing in Unity and googling didn’t really help out. Can anyone explain how to fix this bug? Thanks a lot.

http://forum.unity3d.com/threads/3-2-water-over-billboard-trees.77544/