Wear your bathing suit and make a Hawaiian Lei. If you can't find a grass skirt try making your own colorful version:

Drape the end of a piece of crepe paper over the belt and tape the end to the back of the belt so it drapes over the belt, hiding the belt once it is put on. Don't forget a few ankle bracelets!

Hint: If it is too cold for just your bathing suit, wear tan sweatsuit of tights and turtleneck underneath the costume.