(click pic to see large version)
Sources:
Under the Sea Cake: Super perfect for this theme! Source unknown :o(
Underwater camera: These would make great favors, especially if it's a water/pool party! They have an underwater scene on them.
Sea bath toys: You could use these on the tables as decorations
Fish bowl centerpiece: It has real goldfish in it! The kids would go crazy seeing these swim around as they ate! I think beta fish would beautiful too!
Sea decorations: These would totally set the scene!
Oyster cookies: Simple cookie project that would be fun to eat!
Fish placecard holders: Perfect for holding your buffet tags!
Sea creature bubble wands: What is an under the sea party without bubbles!?! You should even add a bubble machine!
Submarine cutout: The kids will love receiving a copy of their picture taken in the submarine as a thank you gift!
Fish lollipop cover: I love this idea for any theme - just a fun way to dress up a plain lollipop!
Wave cupcake stand: Add cutout waves to all your displays to really carry the theme!
Sea tablescape: Lay a net, starfish and shells to a blue tablecloth to create a fun background!
Octopus napkin: Wrap cotton balls or a small ball with a rubberband and a a blue napkin and glue on some eyes... instant Octopus!
Octopus prop: This giant cardboard prop would be a great focal point and photo op!
Fish topped cake boxes: Same ladies that did the Alice in Wonderland one - I just love their stuff!