Bell bottoms?
Actually, I can't see your picture (not approved yet), but I'd guess that some water climbs up the strider's legs, due to surface tension in the water, so there's a "jacket" of water on them. Or, maybe it's an optical effect due to the deformation of the water where the strider's legs contact the surface. It could act like a lens, which would explain the deflection of light so that you get dark spots (from where light was redirected) and brights spots (to where the light was redirected). I'm just guessing, though, since I haven't seen the picture ... of course, I could still be guessing even then!