Or perhaps, if you'd like something less commercial, but still lots of fun for children, perhaps you could try planning an exotic location party- in fact, why not leave land itself and go under the sea?
For any party, you should think about what you (or the child you are throwing the party for) might like and enjoy, as well as what your guests like. However, some general suggestions for a good underwater themed party are:
Use blue and green streamers to mimic an ocean feel. Hang them all across the ceiling and make it feel like you're underwater! Use colorful fish, lifesavers, and blow up inflatable pool toys around the room and on the walls to complete the look.
You can go for the fancy plates, napkins and cups in an ocean or fish theme. But why not save money here and go with solid blue colored plates, napkins and cups instead? It will still look very "oceany" and will let you spend more on the party favors and other decorations.
These activities are really fun and will complete the under the sea birthday party theme perfectly.
Find some kind of plastic rod and attach string and a magnet (to form a "fishing rod"), take a kiddie pool, put in different pictures of fish or jetsam-type things with magnets on them, and let kids "fish" for a picture.
If you have a pool or can possibly find an indoor pool to take them to, swimming is always a great option! If not, you could always set up a sprinkler or other outdoor water system if you have a yard. Just make sure that parents know kids will need a bathing suit, and if you take them swimming be sure that they can all safely swim.
Games -sharks and minnows: Choose 1 child to be the shark. Kids stand in a line at one side of the room and must run to the other side without being tagged by the "shark". Kids tagged become sharks too. In the next round, kids run back the other way and try to make it to the other side once more- the goal is to be the last one standing.
Goodie bags can include Swedish fish, little toys, puzzles and stationary with watery or underwater designs. Another party favor you could consider are live fish: You can usually find some feeder type fish relatively cheap at a local pet store.
Buy some bowls to put them in and let children take it home with them when their parents arrive! (do not give to child, but let the parent take it with them if they decide that it is ok) So what are you waiting for? Get ready to take your kid and his/her friends on an underwater adventure!