A lot of my scenes include open, shallow-draft boats and I've long been frustrated by the lack of a simple way to create accurate displacements for the hulls. I've tried painted shaders or displacement masks made by hand in Photoshop with mixed success (and it's not much fun).
Here's a technique that (I think) solves the problem. I've tested it a bit and so far it works pretty well. The displacements it creates are precise so there's no manual painting or guesswork. It takes a little advance work but once that's done the same setup can be used for future projects.
First, create a scene with *no* displacement or surface details, just a perfectly level planet surface.
Place the finished boat model at the scene origin and invert it vertically by changing the Y scale to -1. Assuming the model was saved with its origin at the waterline (which of course it was, wasn't it?), the part of the hull that was below the waterline will now protrude above the planet surface and (very important) will be a mirror image of the original.
Set up an orthogonal camera with its XZ coordinates set to the scene origin, facing down directly above the hull. Set up the camera and renderer to create a square image large enough to include the exposed hull. Use round numbers for the dimensions. Add a render layer to output a surface position layer.
Render the layer.
Now, in the actual scene, use an image map shader to place the surface position image at the boat's position and orientation. Set the dimensions to match those used to create the rendering.
Under the Colour tab, select linear data. We only want the Y (green) component of the image, so enable Multiply RGB and set red and blue to zero.
Feed the image into the function slot of a displacement shader downstream (pun intended!) of the water shader. Set the displacement to -1. If the water is reasonably still (low wave roughness) this should make a perfect cast of the hull in the surface.
In the examples here, I included a surface shader to the network with smoothness set to 1 to flatten the water surface before the displacement. The surface position image masks this shader, with a bias scalar node inserted to adjust contrast. Displacement is set to -1.1 to provide a little wriggle room.
The displacement will need to be tweaked -- scale, position, magnitude -- as the the water roughness increases. It seems to work best when the displaced surface falls inside the mesh of the vessel's sides (inside the planking, so to speak). That mitigates the odd reflection and other artifacts that might show up depending on the light angle.
In *very* rough seas this probably won't work at all. But then, you shouldn't be out in that kind of weather in an open boat anyway, should you?
Once the surface position maps are made they can be stored with the models for later use. I didn't do it here, but it's a good idea to include the dimensions of the map in the filename ("bateau111_surfacePos_12x12.exr") so you don't have to remember them. And the basic scene used to create the render layer can be saved and used for future models.
