We've been to Orlando at all those times, and we've been swimming no problem.
Sept/October 2017 - definitely went swimming in the pool
December 2017 - definitely went swimming in the pool
February 2018 - got in the water at Cocoa beach (went for Falcon Heavy launch!)
March 2018 - went swimming in the pool
October 2018 - spent a TON of time in the pool
December 2018 - will let you know!
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Do you like HW or Xmas better? September will still be HOT.
I like November before the week of Thanksgiving OR the week starting the Sunday after Thanksgiving.
That being said, for about 4 years now, we do WDW in early October and we'll do other parks in December if there. Universal is nice at xmas, and you can see the grinch, etc, which is nice.
For whatever reason, to me the theming at HW has more depth.