How to create a simple mobile water inside a 3D container?

My game have bathtub. I want to fill water inside. I was able to do the trick by using a plane and increasing it's height. But the problem is..

Let's say the water inside a container should fill-up an irregularly shaped container. Is it possible to snap the edge of the plane inside the bathtub or Is it possible to cut those or set it invisible trough layers? (X) - I also found http://answers.unity3d.com/questions/228633/making-objects-selectively-transparent-with-layers.html as a related solution but nobody seamed to answer it.