In my opinion, this should be factored into your summer rate, just like increased A/C use, etc.. (Overly restrictive rules about kiddie pools or slip and slides might not be a “positive“ message to convey to your guests.)
If that doesn’t work, perhaps you can come up with your average water usage over a year or two - on the high/generous end - and then charge the guest if it exceeds that amount by a sizable amount (be prepared to show proof). If nothing else, this might encourage them to not leave the water running continuously.
If it were me, I would make a note in my RA or House Rules to please be considerate about water usage. Some people will still abuse it; others will not. It’s a cost of doing business. factored into the overall rate.
Feedback - Water and sewer costs Skyrocket in certain areas of the country ( like mine ) based upon tiered use. The more you use the higher the rate soars.
Even one group using portable pools or slip and slides could push up our monthly bill by hundreds of dollars ( due to our being in a top usage tier because of large house, lots of laundry, lots of showers, large number of guests, plus a pool etc ).
It is really not nitpicky to prohibit portable pools and slip and slides, nor is absorbing that amount of water use a normal "cost of business".
These are strictly prohibited at my homes, without any qualms on my part, and we provide everything generously.
But not this. Water is too expensive for portable pools and slip and slides.
It is spelled out clearly in it's own clause in the contract. Simply put = don't.
Go to the beach instead.
georgygirl1955 has given excellent, proactive advice.
Questions: you say that you were able to determine the root cause by "check[ing] it out." Does that mean that you have security cameras on-site? If 'yes' (and more importantly): do you disclose that they are there + where they are? In your RA -- and -- within your listing?
If there are security cameras > that's your deterrent. But their presence must be made apparent to the prospective/booking Guest. In most (US) municipalities, there are laws that require you to do same.
I don't know how expensive the water is where your vacation rental is located, but I agree with r_and_r that I would figure this cost into your rental rates. Apart from water usage, however, I would not want guests bringing kiddie pools or slip and slides to my vacation rental. One afternoon's use of a slip and slide can destroy the lawn underneath it (this is from the "voice of experience" with my own children). A kiddie pool left in the same spot for a few days can also kill the grass underneath the pool.
I would include a clause in your rental agreement that prohibits guests from using items such as these at your vacation rental.