- Shell pasta with a kid-friendly sauce (mac & cheese, pasta salad, etc.)
- Octopus hotdogs (cut 8 legs into the bottom half and cook so the legs curl up)
- Cut out pieces of different melon with themed cookie cutters
- Dye lemonade or apple juice blue and/or green
- Ben & Jerry's Phish Food (although I'm sure there's cheaper and more creative ideas for sweet things!)
There was a children's event I helped with that did water/ocean/beach themed snacks one day. We used food dye to colour cream cheese blue and spread it on crackers or toast and then put a couple goldfish on top and some "seaweed" cut from fruit leather/fruit roll-ups.
I've also made candy sushi before, although I'm not sure if your little one would understand the reference. Lay a fruit-roll up flat on the counter, spread it with a thin layer of warm rice-krispie treat, lay down a gummy worm or two and then roll it up (like you would a cucumber roll).
Fish shaped sugar cookies would make a great "craft" or activity for small ones too. I'd bake big, fish shaped cookies and make up a few colours of icing. Cover the table with newspapers and let them all decorate their own. The bulk store here sells fish shaped sprinkles sometimes. Just make sure you have plenty of soapy water for cleanup.