I wouldn't plan one for my own child near water like that, because I would be stressed to the max. Worrying about my children near the water and the people, and games, and food, and the party. It would be too much for me personally.
In saying that though, I would probably attend a party for someone else! Especially if the playgrounds are fenced in.
I would definitely specify that parents have to stay with their child/ren. I wouldn't be taking on the potential responsibility of someone else's child in my care drowning.....