What you could do to get exact placement of foam is set your boat with oars at 0/0/0, make a topdown or ortho render (or even a screenshot with the boat shaded or as wireframe) and paint in the foamy areas in photoshop in a layer over that render. Then save the layer as greyscale tif and import in TG, just like you did with the bow wave map. See if it fits, or change size so it fits. Keep image map at 0/0/0 and use a transform shader after it, copy final location and rotation of the trireme into the transform shader, and it should fit again.
In the line of shaders to the lake/water sphere (whatever you use), make a surface shader after the water shader, and mask by this foam image map. Now it should have white areas where they should be. You can further break the image map up, or additionally mask, by a fractal.
I hope this will help you.