Our fingers shrivel "prune" up in water when your skin is in contact with water. When the oil in your skin is gone, the water comes into contact with the dead keratin cells, absorbing the water then cause the skin to swell up.
When hands are soaked in water, the keratin absorbs it and swells. The inside of the fingers, however, does not swell. As a result, there is relatively too much stratum corneum and it wrinkles, just like a gathered skirt. This bunching up occurs on fingers and toes because the epidermis is much thicker on the hands and feet than elsewhere on the body. (The hair and nails, which contain different types of keratin, also absorb some water. This is why the nails get softer after bathing or doing the dishes.)
Its your body's reaction to attempt to keep itself waterproof.Even though you can't see it, your skin is covered with its own special oil called sebum (say: see-bum). Sebum is found on the outermost layer of skin. Sebum moistens, or lubricates (say: loo-bruh-kaytes), and protects your skin. It also makes your skin a bit waterproof. That's why water runs off your skin when you wash your hands, instead of soaking it in like a sponge would.
But staying in water for a long time washes away the sebum. Then, the water can penetrate the outer layer of your skin.
If you are in the bathtub, it's because they don't want to see you, if you are in the pool it is because they don't like your bathing suit and if it is when you are washing dishes, it's because they hate that job!