I've been searching high and low for ideas to "theme" a beach DW that are not seashells or starfish. I figured I wasn't the only one out there searching for ideas so here's what I've come up with:
Tropical Fruit Theme:
Think Citrus or Mango. The tropics are full of delicious fruit - take the "put a lime in the coconut..." and run with it... Here's a few ideas:
Tropical Table Settings - Tropical Table Centerpiece
How to Throw a Tropical Pool Party: Tips for a Pool Party - stylelist
Centerpieces with lime slices

Sandcastle Theme:
So without the seashells, you still have the sand on the beach, right? Why not have a morning of sandcastle building contest on the beach? Provide the OOT bags as children's sand pails... have sandcastle centerpieces. Here's a few ideas:
LIVING : Party Food & Decorations : Sandcastle Centerpiece : DIY Network
2 Designs - Sandcastle Center Pieces
Tropical Drink, "We are celebrating in Paradise" Theme
I posted these in another thread, but I still think they are super cute. You could get some lounge type furniture as well and take the theme over the top.
Fiesta Flowers AZ
That's what I've got so far... anyone have any other ideas?