Assuming the event is more formal than casual, start thinking about your outfit from the feet up. The reason I say this is because you can’t wear heels on the deck of a boat. Soft or light-colored rubbery soles are required (both for safety and so as not to mark the deck). Espadrilles look great with ankle-length tapered pants and a tunic-length top, and ribbon-tie versions look pretty with a sundress; a simple wrap dress in a sleek jersey fabric is effortless and stylish teamed with a mid-heel wedge. Make sure your dress isn’t too tight or too short—you may have to negotiate narrow ladder-like steps between decks, and you want your exit strategy to look as good as your entrance.
