As far as I know it is open for visitors throughout the year, but the excitement is greater if a major sporting event is on. You have to be interested in the American sport scene to enjoy this place; there is a big onus on Baseball (in Spring & Summer, DWWS is the base for the Orlando Rays) but there is something for other sports like basketball.

I think it is not all that popular among Brits and unless you have a special reason for visiting, a day in one of the Water Parks would be much better spent.