I had a fishing party for my son years back. I purchased colorful fishing hats (they were thin and inexpensive and would be great for the beach too)and a fish box for the goodie bags from Oriental trading. The box had to be put together so be prepared if you go that route. I had to tape the boxes together to get them to hold when stuffed. I made a fishing pond and decorated a big box (painted blue, foam fish stickers (I found at Target at the time))and had them "fish" for the goodie bags. I had someone hold the string and make them "set the hook", etc. which made it even more fun. Very animated.
Some ideas that come to my head now after reading your note: fish sticks, Nemo fruit snacks, blue jello with swedish fish (or Nemo snacks) inside or on top. You can make octopus hot dogs and mac and cheese for sea weed.(Dye green if you want) For the octopus, cut a hot dog lengthwise with a thin knife (filet knife works well) to make 8 strips. Keep about one to 2 inches at the top whole for the head. Make 2 holes for the eyes. Then boil in a big pot. The "legs" will curl. I would experiment first since I have found some brands/types of hot dogs would curl better. It has been a while so I do not remember which one I used. Keep some weiners whole because some kids will not eat them like the octopus, which really surprised me since I thought they were fun. I guess every kid has their quirks. HAHAHA
For a sea life party I did a few years back, I went to Houston Balloon and Novelty and found inexpensive blow up sea creatures (whale, fish, shark, etc.) Dolphin key chains. Sea animal water guns. I think it was Oriental Trading (mail order) that I found a whale pinata. I think I have seen a Nemo pinata through Birthday Express (another mail order co.)
Hope this helps. Good luck and have fun! Happy Birthday to your little one!