Create a map in Photoshop with the shape of the lake, and use that for the alpha of the water material?

Export the map to TIFF with transparent background. In Vue use the function editor to import and connect the alpha of the Tiff with the alpha node of the water material (You probably have to disconnect the connection from the Color node to the Alpha node first). This should make the edges transparent, while the global transparency is still used for the water transpsarency.
Don't forget to set the material to object parametric and use a plane (not infinite) as base for the water.

Cheers!