Hi Amy,
Good news, you don't need to add the
Water Park Fun & More Option now; it can be added once you arrive at Walt Disney World and know how the weather is. In fact, you may want to compare the cost of purchasing this add-on to your tickets compared to just purchasing admission to the water park. Depending on your plans, it may be cheaper to do the latter.
With this in mind, December weather can be unpredictable in Orlando. While we were there just last week, we had comfortable days in shorts and other days in jackets/pants. This year during December,
Blizzard Beach was the only water park open as
Typhoon Lagoon was undergoing refurbishments. We debated visiting Blizzard Beach on one of the warmer days; but decided while the water is heated, we weren't brave enough to handle the time outside of the water moving from attraction to attraction. However, I know there were guests at the water parks, and the crowds were light. This may make it very enjoyable!
So if I were you, I'd wait to purchase the ticket add-on (or water park admission) until you actually arrive at Walt Disney World to see how the weather feels to you to see if you'd feel comfortable at the water park.
Have a great trip, especially revealing the big surprise!! Please let us know if we can help again.