To be fair, you're definitely wrong about sand, though the gravity effects would  definitely play their part dry sand is immensely more annoying to move in, and the gravity would probably be equally problematic for both dry and wet sand. I can only rely on 20 years of sporadically visiting beaches, but I have never once seen wet sand (not sand actually underwater, but sand that has been saturated to some degree) be more difficult to walk through than dry sand.