When your skin wrinkles from water, it's not because you absorbed it. It's because of a nervous reaction and it can be turned off!

We've all been there. We spend too much time underwater and our fingers start becoming all wrinkly. You might've thought that it was because your skin absorbed all that water. You'd be wrong.

The reason for this is because the wrinkling provides us with an evolutionary advantage. Apparently the wrinkling gives us better traction in wet conditions. And it seems the only thing water does is trigger the process for our skin to get all wrinkly.

Researchers noticed that patients that had some sort of nerve damage in parts of their body wouldn't get the wrinkles. However, the parts of their body that weren't damaged would get water wrinkles. More recently, it's been found that water initiates the wrinkling process that leads into a synapse of the blood vessels underneath the skin, which then constrict and decrease the amount of liquid under the skin. This causes the wrinkles.

(Source)