[elric]

I can't vouch for the veracity of this claim, but I once read somewhere that it takes a while for the vitamin D, which is apparently created in the upper layers of the skin, to work its way down into the bloodstream, and that if you spend a lot of time in the water, it doesn't make it through. Maybe because being wet a lot tends to dehydrate the skin? I don't know.

[mikestew]

How much skin is exposed when wearing a wet suit? There's your answer. Just depends on whether the study took place in California (which has chilly water) or Hawaii (which doesn't).