This is the correct answer. One or other of the water parks always closes at the end of October, then the other one closes around January time. They are usually both open again around March/April.
My favorite is BB, but we'll go to either. What do you think the odds are of us enjoying one last trip on our PAP, in December? I have scheduled one day with late in the day FP+s just in case it is nice enough.
BB has more extensive maintenance this year and they had to close it three weeks earlier than usual. They might close the one having the most maintenance first, so they don't run out of time completing the work. Then they can limit the maintenance on the other one to get it open on time.
I have been a DVC member since 2010. I usually go to WDW in December. It seems to me that Typhoon Lagoon (my favorite) is always closed then. However, this year, since I'm going in January . . . figures . . . Typhoon Lagoon is open in December, and closed in late January when I'm going.