One of the many hats I wear in my professional life is logistics and project management so I see everything in terms of time spent and the steps you have to take from point A to B.
Generally every time you fly it takes about an hour from the moment the wheels hit the runway until you're out of the airport. This varies obviously but it's true if you're sitting in the middle of the airplane, shave off 10mins if you're in first class, add 10m if you're in the back of the plane. Adjust for size of plane.
Probably 1-2m per row.
Then you have to deal with transportation. Depending on how good you're with timing an Uber ride, it's 5-20m waiting for an Uber. I haven't done any of the bus services post Magical Express but that could be 1-2hr added.
Typically you're looking at roughly 1.5-3 hours from touch down to get to your Disney resort. So if you want to use this perk you really need to land before noon just to get to your resort with enough time. Then maybe another hour of getting ready and getting to the water park.
The caveat is after all that travel how tired are you? Do you have kids? Elderly family members? Is it worth all the rush and hassle of getting to the park after 8 hours of travel?
If Disney really wanted to get people in to the water park, they should just give everyone a voucher to get in to the waterpark after 1pm? 2pm? free once per trip and leave it at that.