I doubt much of the water is wasted, though a tiny bit of spillage and a bit more evaporation is expected. The rest is pumped back to the top, as it would be hard to source fresh water fast enough to just waste it.
I wouldn’t say that the water loss is tiny. I’m assuming average temperatures between 90-100° F when operating and low humidity (<30%) and I found a source saying that the city of pheonix estimated that an Olympic swimming pool loses 2500-5000 gallons of water per month. That has mostly to do with surface area but I’m assuming they have at least one of those pools in the park, maybe multiple. Could be up to 15,000 gallons/month just for pools alone.
Then we talk about the water slides which, if you’ve ever been on a water slide, you know that they waste water. Water leaking from the slides may be gallons an hour and the agitation of the water will speed up evaporation. I’d say each slide loses somewhere around 10-20 gallons an hour as a guess. You multiply that by 15 slides and you’re getting 150 gallons per hour, 12,000 gallons a day.
So not to put it lightly but this one park could be losing over a million gallons of water a year. Easily. And in a desert that’s nothing to scoff at.
I did all my math before researching but I found an article from the guardian that uses very similar numbers. Mine are higher but this park is also bigger.
Better, in fact. You wouldn't need to treat seawater as much. If you're anywhere near an ocean, you can just flush it back every few days and refill. Cruise ship swimming pools are done this way.