Where the ocean meets the shore

How would you tell where an ocean meets the shore.
Lets say you have a plane, as the water, and a terrain as the island, and you want to place a foam texture where the ocean meets the terrain. Is there any way to do this?

Height.

The ocean meets the land when they are at the same height

Ok, but how would I put the foam texture there?

Maybe instead of a plane for the water, you could use a custom mesh (like the one in the 'flag.bb' example) ? The advantage would be, that you could set the texture coordinates for each triangle.
You could then set up the foam as a second texture layer, with it's own TexCoords, and then use PositionTexture to move it.