OK, Go to the part of the "Surfing Wars". When one of the girls (Abby or Maggie-I forget who) goes out to go surfing, she gets her hair wet surfing, but in the next scene when she's talking to the two boys and her sister after she's done surfing, her hair is dry again.
it is because they had a stunt double do the surfing and the only thing that Mary-Kate ( I think it was her) was paddle out and she didn't have to wet her hair for that. Also you can tell its her stunt double because she has a different hair cut also you see her later while they all limbo.