R.J.
When I lived in FL I wore wool socks, gloves, layered pants/tops in December... Because I was FREEZING.
It was 71.
That's swimsuit weather in Seattle.
Honestly, FL winters are like most places early fall or late spring. There will be a few days in the 80's, a few cooler days in the 70s. A random 'cold' day, and a random 'hot' day.
In the oh-so-frigid 70's the water is all heated.
Although, coming out of a NewEngland Winter... The kids may be sweating while the G's are stacking coats on... I'd still just let the G's use their judgement on where a day is perfect for, or too brisk, for water play.