It doesn't really matter what the outside temperature is as long as you are comfortable with the water temperature. I have swam in an indoor/outdoor pool in the middle of January in below freezing (in Canada) where the water was probably 90degrees. I even stood on the deck for a few seconds and was still warm with the water steaming off me.
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About 85F - up enough to be warm when you get out and under 90F if it is windy will freeze you