I went to a carribean fancy dress which is kinda similar. I wore a bikini top with a boob tube over the top so i wouldnt freeze, pair of jeans with sum flip flops and stuck a flower in my hair and wore sum shades. Its quite easy dress up really cos its all stuff you'll already hav in ur wardrobe.
Or you could do what my m8 did n go as a life guard, yellow polo shirt, red shorts, whistle and water wings for comedy value and a buyancy aid if u hav one