Wanted to add some more information after researching the water park costs in our area.
Hawaiian Waters (2 owned by Premier) and Hawaiian Falls (3 owned by Ripley's) are the best indicators of what will be similar to the Universal expansion. They are "Micro water parks" that range between 9-14 acres and cost $5-15 million 15-20 years ago. So with inflation and increased construction costs, that number would be more like $10-30 million today. They are also city owned and leased back so they surrender 6-8% of their revenue back to the city. So that's a pretty fair construction cost as land isn't being bought in those costs.
They generally have tall towers and slide complexes with lazy rivers, etc. Pricing is $25/28 per person or $40 for an annual pass and are open ~80 days a year. All summer, just weekends once school starts, then closing end of September.
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Universal has much less area to work with for their largest expansion plot located behind Trolls and the north parking lot on the northwest side of the property.

During the initial zoning and site plans, the proposed area was a little over 5 acres. Obviously the original site plan has changed, and overhead google measurements place it closer to just under 7 acres.
Note the 2 Parking lots on the left. Orange would be the water park area. The green is a current walkway that would allow in-park entrance from between trolls and Shrek to enter the water park area. Blue could be a walking path from hotel to the water park entrance.
Not sure what the budget for a ~6-7 acre water park would be- but it couldn't possibly be more than $50m. Likely not even half that at $25m considering the size, lack of lazy river (or minimally sized one) and lack of large slide structures. And if tickets were $15 more for water park access, that would be 1.66 million in sales to pay for the construction @ $25m. According to the original release from the city zoning- The park is set to accommodate 7,000 visitors on weekdays and up to 14,000 on weekends, with a peak capacity reaching 20,000. So a conservative estimate would be 3m people.
Which shows just how profitable water parks are and why they are almost always the first expansion at any regional park.
It more than pays for itself within the first year of operation.
Let's get those shovels in the ground, guys.