I googled the title of this thread, "What happens to your skin if you stay in the water for weeks?" and I found the following reply on the internet: My reaction to reading this was ... Is that true or was the person that posted this just making it up?? I'm guessing the latter, but what do I know??
The osmotic potential of the skin is delicate and the constant pressure and presence of water could burst cells, tighten tissues, yeah it could possibly happen. We're not aquatic creatures, being in an unnatural (to our skin) environment has it's problems.
If you're lost at sea for weeks and you haven't starved to death or died of dehydration, chances are you have some kind of vessel that would be capable of keeping your body out of the water.
Do you have any idea how many Aquamans died before they finally developed a decent underwater skin protectant? The sea is littered with skeletons in orange body shirts and green tights.
I have to imagine it's the same principle behind bed sores or how people in wheelchairs sometimes get blood clots from sitting for too long. Unnatural pressure for long periods of time is bad for you.
The technical answer is that something called maceration occurs. Maceration is the disintegration of skin when in prolonged contact with water. This is a variant of the normal wrinkling/swelling/softening that happens to your fingertips if you've been swimming or in the bath for a long time, but spread over the whole body. Left long enough this destroys the integrity of the skin, causing it to fall apart at the slightest touch. You will also get a chemical change in subcutaneous adipose tissue in any anaerobic environment (water is anaerobic enough for these purposes) that is akin to saponification: essentially, your fat turns to soap through hydrolysis. This takes longer than maceration, but will happen eventually. Other bad things happen too, but I think that's enough for now...