I don't think it is ridiculous at all. We go to water parks many times each year, including several times after the pandemic began and had no problem with putting on a mask for the restroom. We always would stop back at the loungers to towel off and put on shoes before going to the bathroom anyway in pre-covid times, so it really wasn't that much of an adjustment. But even if it was, the fact we've always stopped by the loungers on the way to the restrooms even before we had to is proof it can be done without much inconvenience.
I do expect that if people are not complying, they will post someone at the restroom entrance to require it. Don't see why they need paper masks handy--to buy a ticket you have to agree to the rules, including that you'll bring your own mask and wear it in the required areas.
I don't put too much stock in people on facebook saying something is nuts. There are a fair amount of people on facebook who are convinced it is impossible to breathe while wearing a mask and doing so in the Florida summer would make you pass out, despite the evidence of innumerable people who manage just fine. Seems to me the people who have the hardest time with mask rules are the ones who give up and declare it to be a nightmare before they even give it a fair try.