Understanding DepthMask


I’m new to unity and I’m trying to mask parts of my game-objects with the DepthMask, which seems to be the only possible way to do it:

I downloaded the example file, which is great. But even though it’s not much code, I don’t understand how it works.

Example File: http://dl.dropbox.com/u/6231276/Depth%20Mask%20Example.unitypackage

What I want to do is to turn the masking around: In the example file, the boat is masking/cutting off the overlapping part of the water-plane.
I want the water-plane to cut off the boat.

Does anybody know how to achieve this effect?

It would be a huge help. It would also help me to understand how to use the DepthMask.

I got it working based on the answer here: http://answers.unity3d.com/questions/56564/Treating-an-animated-sprite-as-a-mask.html