When you are immersed in water for prolonged periods the dead, keratin-filled cells from the outer most layer of your epidermis (called the stratum corneum, which acts as a barrier between the body and the environment) absorbs the water. This, of course, causes the stratum corneum to occupy a greater surface area. Because it is attached thoroughly to the tissues below (and they haven't changed), it must wrinkle to account for the greater surface area. The stratum corneum on the palms and soles is much thicker than that on other parts of the body and so absorbs more water and makes the wrinkling more evident. When you see your fingers and toes wrinkle up after long exposure to water rest assured that the skin is just doing its job of intercepting water that would otherwise invade your body by an inappropriate route. http://en.wikipedia.org/wiki/Stratum_corneum http://biowww.clemson.edu/biolab/wrinkle.html

The raisin for the wrinkles is because of a protective layer on the skin called the callus. The callus is a layer of cells thicker than our regular skin cells, the cells in the callus themselves are 'dryer' inside than other skin cells and have thicker walls . They are practically dead . The callus and the individual cells are not water proof so when you get soaked the cells and the layer absorb water and swell up. The callus layer doesn't stretch like other skin, but the water softens the cell walls and the wrinkles form to give room for the swollen cells. The softening of the cells is also why people who try to remove the callus with emery boards do it after a bath. ( Sanding the callus may actually be counter productive, the friction and heat stimulates the skin to form more callus, especially on dry skin, the water at least acts as a lubricant and coolant.) The callus is a protective layer that forms in response to friction and wear on the skin so it is found mainly on the hands and feet. People who use their hands a lot, carpenters, gardeners etc have a thicker layer, and as we get older it gets thicker, but everyone has at least a thin layer on the hands and feet, it forms naturally. Even a callow youth has a youthful callus. That's why we usually only see the wrinkles on our palms and soles. Elbows and knees also have a natural callus layer but unless you do a lot of crawling or have some weird job involving rubbing things with your elbows the layer is thin and the wrinkles don't show up as much. People who keep their noses to the grindstone could wind up with a wrinkly schnoz. Unfeeling people with a callous heart could wind up with a wrinkled soul.

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Water is absorbed by the outer layers of the skin, which consequently swells and wrinkles.