A New Story This Week notes that one in five Britons would be willing to go completely bare on a beach IF they were out of the country on vacation. Only one in twenty (5%) would be willing to go bare while in their home country.
The "what if someone I know finds out" factor seems to be significant in the U.S. too. It may explain the popularity of nude cruises, where people know the ship is traveling out side the country. Bare Necessities often reports a significant number of first-timers to nude recreation on their ships.
By the way, the link above has other interesting factoids on the page, such as data on nude sleeping. It's worth a visit.