You can buy a glass fish bowl and fill it with blue jello. When the jello starts to set put gummy or swedish fish in it.

Very cute.
The blue koolaid idea is cool, but I would recommend cups with lids and straws so no children got choked if they 'drank' a fish.
Have fun!
Here's some ideas from
http://www.sanjosefamilies.com/finding-nemo-birthday.html
Of course, you HAVE to have goldfish crackers! Only, call them Nemo crackers.
For a cake, you can create a simple fish shape by cutting a 9x13 sheet cake. Or, take one of your Finding Nemo napkins down to the local bakery to have the image "painted" onto the cake. Or, for those who want something truly awe-inspiring:
Finding Nemo Aquarium Cake - Bake a 9x13 cake in any flavor. Cut out the center so that you are left with a big rectangular hole in the middle. Next, make blue jello using the Jigglers recipe. Pour this into the center of the cake, and place gummy fish (also known as Swedish fish candy) into the jello. Frost the cake edge as you like, and frost the part you removed from the center (you can serve it too). Chill the cake so that the center firms up. Serve and cut, giving guests the choice of cake, jello, or both!
Octopus Hot Dogs - After cooking the hot dogs, cut half of it into 8 lengthwise strips (or whatever you can manage) but keep them attached to the uncut half of the hot dog. You're left with an uncut "head" and cut "tentacles" on the bottom. Yum! You can put them in a bun or not.
Starfish Sandwiches - Make tuna sandwiches and cut the bread into star shapes. Easy!