ship trail on water

I have done this before using a dynamically generated mesh, which is flat and spreads out in a V-shape behind the ship (or in my case, it was a jetski) with an image of the white foamy parts of the waves (and transparency so it blends with the normal water texture beneath).

You would do this in a similar way to the way that dynamically generated skidmarks are created for racing games, execpt that you would want to allow the "trail" to gradually expand of its own accord to simulate the expanding wake of the boat.

If you've never done dynamic mesh generation before, I would say your best bet would be to take one of the existing skid trails scripts out there, study it a bit and work out how to make it expand gradually over time, and apply a transparent foamy texture to the mesh instead of the dark skid texture.

There is a skid trails script in the new Car Tutorial on the Unity site.