If there's availabilty on the following night, they sometimes slip a special second night offer under your door shortly after your arrival. We've gotten that offer several times when the second night would have been a sunday-thursday. It's been between $99 & $129 a night depending upon what room type we were in.
Of course, the second night offer is never guaranteed, but you could plan on staying until 10 pm waterpark closing time and book an inexpensive reservation for the additional night that can be cancelled without penalty at another nearby hotel just in case you're not offered the second night deal.