Unless it is very chilly, Disney keeps at least one of the Water Parks and the resort pools open all year long.
The water is heated in most pools.
If it is sunny with the temperatures near 60 degrees or above with little or no wind,
guests can be found swimming at the resorts and water parks.
No need to buy the WP tickets in advance, as you can add them ("Park Hopper Plus/Water Parks Fun and more") to your tickets after you arrive at no extra charge over buying them in advance.
(even on the day that you do decide to go swimming.)
You can wait to see if the weather suits you for swimming.