Professor Richard Guy, Professor of Pharmaceutical Sciences at the University of Bath, helped James Tytko with the answer…

Richard - Thanks James. The truth is, water can penetrate the skin from inside-out (and outside-in, for that matter, but less so) by passive diffusion. The water concentration inside the body is about 50 molar; outside, it’s much less, meaning that there is a strong driving force for water to ‘escape' from the body especially as an adult body’s surface area is 1.5 to 2 m^2 (15,000 to 20,000 cm^2).