I have managed to acquire a ton of fish serving pieces over the years, and I put them to good use in June, July and August. Not always for food, either: photo above is from Keno at my house, which had a summer theme. Those are the game chips in the fish dish to the left.
One of my favorite tables at Reading Rocks was inspired by Dan Yaccarino's "Jacques Cousteau":
They rented the "Ghost" chairs, which I thought was a nice touch. I also liked the snorkel masks at each place:
If you had a giant Barbie doll hanging around (I don't, but if I had girls, I might - who knows?), you could do something like this for a centerpiece: