Where do you come up with these questions? You discussed stranded on an Island my sister posted that months ago and there were many hubs written on it, start writing and research your questions they could make good hubs.
I cannot take credit for this answer, since I used Google to find the answer. I know, I know... so hard to use a search engine to find the answer, right? Anyway, this seems to be the top answer, although there are other theories:
The outermost layer of the skin swells when it absorbs water. It is tightly attached to the skin underneath, so it compensates for the increased area by wrinkling.