We find that when we visit the water parks, we like to arrive when the park opens so that we can get on some of the more popular attractions right away to avoid the long lines. We generally can get quite a bit accomplished and since you are visiting during a less-crowded time, you may be even more successful having shorter waits. Our visits have usually been longer than 3-4 hours - more like 5-6 hours (my kids are 15, 12, 9). However, our visits are in the summer with bigger crowds.
Typically, we begin wrapping up our day at the parks around 2 in the afternoon (we usually eat lunch at the water park & treat the kids to donuts made fresh at the park) because we seem to get more tired at the water parks than the main parks. We then freshen up back at the resort & then go to to a park, much like you have planned.
So in a nutshell, I think you have a great plan; but, I'd just be flexible in case your kids decide they want to be there a bit longer.
Happy planning for a second trip!