Look at the seat and ground of the row you are getting in on the boat. If they are wet, you are probably going to get wet. If they are dry you are good. I am that Mom that saw that one side of our row was pretty wet- and quietly avoided that side so the kid or DH would get it instead-- and they did. I would say 90% of the time, not more than a few drops. That other 10%, I have had some big wet spots on my pants, and I do remember getting soaked once, but that is unusual.
We have been to WDW in early December many times. It might be 90 degrees for a high, or it might be 50 degrees for a high-- and you could get both in the same trip. It is unpredictable and the weather reports can be wrong- pack for anything.