[raphlinus]

I am not an expert on this, but I suspect that the "diffuse reflection" picture is not telling the whole story here, as it shows reflections purely from the surface. From what I've seen, in most diffuse materials it's subsurface scattering that dominates. So I think the main contribution of wetness is that it decreases the internal scattering because the indices of refraction are more matched. The absorption remains similar, and I believe reflectance is mostly a function of the ratio of scattering to absorption (Kubelka-Munk theory).