Under the Sea Theme party ideas for a five year old?

I'd like to have something unusual for a change for a birthday party instead of the usual handing over of kids to your local fast food diner.  Any suggestions?

I came up with the Under the Sea idea. It will be in the late Spring when the weather should be fairly nice. I'm thinking it will last for about three to four hours in the afternoon or perhaps more. We have plenty of space outside and inside for the kids to play and run and I'm up for making themed indoor and outdoor decorations that are a) cheap and b) easy to make.

I want this to be memorable! Suggestions on themed food and party games would be great too. What I don't want is to stick them in front of a TV for ages.

There will be about twenty kids between four and seven. I'll have at least six adults there I can count on to join in the fun and help with preparations. Thanks! All suggestions big and small are welcome.