We usually go for around 9 nights in the summer. We either have had APs or get 10 day tickets with hopper. We will often go to most parks twice.
Sometime we may just go to Epcot for dinner and want to be able to enter the park if needed or we may want to watch fireworks in a park after hanging at the resort all day.
We may spend a day or two at the resort and not go to a park but want to be able to change our minds and will often get FPs at a park just in case.
We have only done a water park once and it was not really my thing, but the kids really liked it.
Hope you have a great time!