Are you referring to the temperature of the pool water or of the air? Not that it matters but just checking...
For our family, it would have to be mighty cold before my kids would not swim. Coming from Central Wis for a winter or spring trip, they are just happy to see water that isn't frozen. The "coldest" time we visited was during a trip in late Jan/early Feb and we used Storm Along Bay daily (much to the surprise of the lifeguards wrapped in sweatjackets and pants

). Of course, our schools were cancelled the day before we left due to below zero temps so anything above 32 degrees was balmy to us

rotfl2

.
I want to say most days were 50 degrees or more and there were a few other brave souls swimming as well...