Actually it's proven that nudity does help with swimming - lowers friction and drag. There's even a theory that the short fur on humans (we actually have just as much hair as a chimpanzee - it's just shorter) is because we were "water primates" half the time, and tree primates the other half. Not sure how much stock is in this theory - but it stems out of how well we adopt to water and, of all things, the bouancy in human mammory glands.
I wasn't talking about running, but it would likely help a little there in terms of wind resistance and a well toned athletic woman will be unlikely to have a large chest. As for men, Greek warriors proved you can run just fine nude - some of them had running ability unmatched to this day, and they did it nude, running for a day or more non stop.
If nudity is a cultural norm, modesty is not an issue. Modesty is an artifical creation bred out of induced shame over one's natural state - you don't find it in the traditional cultures that don't cloth themselves.