I found an explanation for my question here http://www.pitara.com/science-for-kids/5ws-and-h/why-do-we-have-wrinkly-fingers-after-swimming/, which suggests that water washes out sebum (special oil, which covers our skin) and water gets into outer layer of now unprotected skin by osmosis. This article also suggests that fingers are not really shriveled - they are water-logged. But is this right? I think I observed many times my wrinkly fingers after swimming, but I never felt that they are water-logged. Any thought on this matter would be highly appreciated.
Great question, for which still no definite answer is known despite ~100 years of research.
A simple physical basis for the wrinkling however seems unlikely as glabrous skin without sweat glands does not wrinkle, and as wrinkling requires the activity of specific nerves.
Moreover plausible evolutionary ideas, which used to be popular, such an anticipated improved ability to grab, don't appear that clear in newer experiments.
For the information above, and an even more detailed answer, please see Haseleu, Omerbasic et al. 2014 http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0084949