You will stop at St. Maarten and St. Thomas before then. Believe me, there are dozens of storekeepers in these ports who would love to sell you the perfect attire for the party!
Guys wore a tropical themed shirt with shorts or slacks. Women wore tropical themed sun dress or again, the tropical shirt with shorts/slacks.
We didn't pack anything for this night (forgot all about it), but found very inexpensive clothing on St. Maarten for this occasion.