We were just there December 15-26. When planning the trip, I had the same question you did about whether we'd be able to do the water parks. I planned to purchase the tickets ahead of time through Ticketmania to get a discount, but I wasn't sure which tickets to buy: the base ticket, or the base ticket with plus options. I ended up getting 8 day base tickets figuring that I could add the plus option once I got there and saw the weather forecast for the week. I'm glad I did this. It turned out that we did have several very nice and sunny days, but the temp was only lower to mid 70's. It was fine for us to use our resort pool, but probably would have been too cool for the waterparks. So that saved me $45 a piece by not getting the plus option.
Another reason I wanted the plus option was so we could go to Disney Quest. But while at Disney, I made a call to Ticketmania to purchase DQ tickets and they delivered the tickets right to our resort the same day for $10. The DQ tickets were about $20 each, which was quite a large discount from the door price, and still less than the $45 plus option.
I also did not purchase the park hopper option, which was another question I had when deciding which tickets to purchase. This ended up being a good choice as well. At MGM, we were there the entire day (2 days) because there was so much to do, so hopping wasn't necessary. With Epcot and MK, we went in the morning, went back to our resort for an afternoon break, and then returned to the same park for th evening. For Animal Kingdom, we were there from 9-5, and then spent the evening at our resort, too tired to go anywhere else. We never felt the need to go to more than one park in a day.
This is kind of more than your question asked, but hope it helps.