Rough surfaces appear darker in sunlight when covered with water due to the way light interacts with the water and the surface. The polarization of light and total internal reflection may play a role, as some light is reflected off the water's surface and some is absorbed by the denser medium. This effect is influenced by the angle of incidence and refraction, which affects how much light is scattered versus reflected. The presence of water reduces the scattering of light that would typically occur with a dry, rough surface. Overall, the combination of reflection and absorption leads to the darker appearance of these surfaces.