Hi Brocklesnar69 - Yes, Disneyquest is open in January. We've been to WDW several times in December/January and the weather is variable. We're going again this year (Dec 28 - Jan 07) and will wait until we get there to decide whether or not to add the "water park fun & more option" to our base passes. If all goes well we plan to hit Typhoon Lagoon early in our trip and Blizzard Beach on the 7th of January. Both parks are great, but Blizzard Beach is our favourite (family with kids who are now 10 & 13). If you're only planing to do one of the "more" options, it's cheaper to pay for it separately at the gate.
Cheers, and have a great trip!