We keep the water in all the slides and pools around 80F (27C) except for Shark Reef which does not have filters, pumps or heaters. What you want is a day that will be bright and sunny and preferably not windy.
If you go on a day that is heavily overcast you will not be able to take advantage of any radiant heating (from the sun) or on a windy day (especially with a dry wind from the North) you will get hit by wind chill.
But a day with the highs in the mid-50s or above which is sunny and calm most people (at least from Northern climates) should be comfortable.
I have seen them close the water parks early. A couple of years ago we only had 25 Guests in the park at 11:00am, an hour after opening, and actually the temperature was expected to drop and it would get windier. A decision was made then to close at Noon. If anyone else arrived we let them through the gates as long as they showed a ticket. It turned out that every person who had entered the Park had multi-day or annual passes or were Cast Members getting free entry. If there had been anyone with single day tickets we would have given them Rain Out tickets because of the early closing (and our three hour rule).
And I had another time when I was called at home and given a choice to either go to WWS or not report in as they were not opening that day.