What I've humbly suggested is, instead of pocketing the extra money, spend it on building more slides. It may take time, but you'll eventually get even more kids paying $20 each, and having a lot more fun.
That's only worth it if people pick water slide parks based on how busy they are. If they mainly just select based on price and the 'Unlimited' feature then there isn't any point in building new slides.
Hell, if the people are mainly just interested in a cheap price and hearing the term 'Unlimited' maybe it's a better business strategy to spend the extra money on advertising your already overcrowded water slides!
You've now made a lot of money with little investment. What if someone else starts building an alternative water slide park?
Well you could then add some extra slides to your park and use your stockpile of cash to give away unlimited slides for free, announcing this on the same day as the other water park opens. You then run your water slide park at a loss for a few months until the competitor goes bankrupt at which point you may choose to pick up that second water park at a big discount. You can then put prices back up to $40 a day to compensate for the free days you had to give out before.
Eventually you run all the water parks in the world, you charge whatever you want, move your headquarters to a tax haven and use the money you've saved to offer large contributions to political parties in return for dropping any lawsuits against you.
It's the American Dream.