For fun centerpieces you can get Beta fish in bowls with shells (real or fake) in the bottom or make beach scenes with playground sand (cheap at Home Depot) and guests can take the fish home when you are done, they are only a few dollars each...

It may be too late for the OP's party, but when I planned a reunion party last year of our large wedding party in Antigua, I used www.orientaltrading.com for a lot of decorations. Great prices on the ubiquitous grass skirts, palm tree toothpicks for the finger food, umbrellas for the drinks, tiki glasses, etc.