the easiest thing to do is to pour some 50/50 antifreeze down the drain through hull hole(s) on the side of the boat. You might have to pour a gallon or two depending on where the pumps are located.
MP probably did that already, just an FYI for anyone else that wants to do it. The pumps could get a crack in the pump body if there is water trapped in there. I would think that the water would expand down the hose rather than crack the pump, but I have heard of it happening.
I generally take the sacks out, just to let the carpet dry out. My ballast system has a weird configuration so I only need to winterize the drain pumps, and I can do that with the sacks out.
