To watermark a mesh is actually too simple. There could be many "secrets" hidden in a mesh that probably only its creator could know well. From its geometry to the uv/texturing process and composition layers and rendering/lighting environment...a mesh theft has no way to rebuild your lighting/rendering physical environment, he can only get what you've already done--the mesh model and baked textures, but he cannot copy the GI, the lights/sun, the materials and reflections that you created and simulated to produce this mesh's texture, and moreover, the process of this geometry's birth can also be stored in some 3d programs like 3ds max and maya. All of those are evidence that you're the creator of that mesh. Futhermore, as for the mesh (geometry) itself, some deliberate build or design or snare in the geometry can neither be detected by 3d software nor huaman eyes, only when you point it out, it'll then be discovered. Unless that theft tries to deconstruct it, but it's impossible to know where to start deconstructing because a mesh contains thousands of vetrtex and faces. And if he wants to deconstruct the mesh, he needs to construct it back, and nothing can be more stupid than this. Not to say that reconstruction will break origial geometry data, and the work itself could be even much more difficult than building a whole new mesh by himself.
There're just too many scientific strategies that you can do in every stages and in multiple ways to torture a mesh theft.