B.K.
It won't be much of a party at a water park. Everybody will be going their own direction. Having it at home or a nearby park with just some cake and light food would be a good thing. (Yes you would have to pay for everybody's entry to the waterpark if you had a party there. It would be rude to invite people and expect them to pay.) Babies don't always last long at their first birthday party. Keep it simple and at home and invite family and a few close friends.