I suspect that some amount of moisture is normal. You might try it again with thin poly liner socks. I've found that all the breathable membranes transmit more moisture if they are kept closer to the skin where the water is more rapidly converted to vapor.

Also with plastic directly outside the Sealskin any water that is transferred has no place to go. They are designed to keep water out but I doubt they transmit much water from inside to out when they are functionally submerged.