That kind of park picnic sounds awesome! This is a county park, so it's not nearly as fun as a state park, but it does have the water park attached. Since it's county run, it's not quite as pricey as a regular park. Kids are $5 and adults $10. But still, with my parents and all of our kids, that's 3 adults and 4 kids (my middle son is with my sister in another state, or it would be 5 kids), which isn't chump change.
I think this party is less rude than it could be, because the parents that are hosting it let it be known upfront that the water park admission isn't included. The invitations were word of mouth, and when she told us, she made clear that part wasn't covered.