Diffusion will occur until the concentration of that molecule is equal on either side of the skin. The sea is composed mostly water, but dissolved in the water are also various salts. The living tissue of human beings and most fishes are considerably less salty than this. As a result, there is more fresh water inside the human or fish than outside in the sea. In response, water naturally diffuses from the body across the skin, as though attempting to dilute the outside sea.
Human skin is relatively water-tight, but fish skin in rather leaky.
As a result, the bodies of most marine fishes are constantly losing fresh water to the surrounding sea. But all living things need a supply of water inside their bodies in order to function properly. But sharks have hit upon a different strategy. As a result, sharks do not continually lose their bodily supply of freshwater to the sea. Instead, any fresh water they need diffuses gently into their bodies through the mouth, gills, and other exposed membranes.