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.