You would think that Mary Kate's hesitation would have something to do with being unable to swim, but it doesn't. The reason she refuses to swim in the ocean or anywhere even though she would love to, is because it's asking for a bikini shot from the paparazzi and she doesn't want that.
“I don't need to be on a 'Who's Skinny, Who's Fat, Who's Looking Healthy, Who's Not Eating?' list.”
She went on to say that if she sees a pap coming she will do everything she can to hide her face so they can't use the photo. The only time she will happily pose is for premieres, openings, award ceremonies and when she has something to sell or promote, or her sister has something to sell or promote. Actually she just said the first three, but come on, who is she kidding. If she has something to sell or promote she's out there like everyone else doing what needs to be done.
I do feel for her on the bikini thing though. You know they would be plastered everywhere, each with their own headline about why she was dying, or her weight loss or some other kind of crap.