Martin
A heated garage is a plus. That is also my situation, my boat is literally in my living room.
When I return home from a day on the water I put my motors in the trim down postion, this lets any ice build up inside drain out.
Also most of your water lines in the boat, will have some water sitting in them. I have frozen my T connections before, no big deal but they have to be fixed.
I have tipped my rig up on cement blocks to throughly drain everything.
Then like it has been said do not use bilge or livewell pumps.
One of my biggest pet peevs…is the preformance at the RAMPS. I can not tell you how important it is to let your trailer drain at the rivers edge, not on the ramp. If you are not at Everts you had better carry some bags of salt.
It only takes one guy that does not drain his trailer. And the ramp is an ice skating ramp. And I have seen many a rig pulled into the water. So check the ramp before you back in and do it slowly.
Another thing when it is this cold..Guys use heaters in the boats. This can be helpful to warm up and to cook soups and hotdogs for a warm up lunch. Just use extreme care.
Fire and boating do not mix well.
Make sure that there is plenty of room around your heater and check it often.
I have had people get to close and burn there gloves and jackets. Alot of this new fabric on the new suits melt very quickly.
Another point I would like to make is your reels can become frozen and very hard to reel. Take em apart and lube with a light oil, if you have problems. I always have 6 to 12 rods pre-rigged and ready to go. I have found some of my newer reels with heavy lube, needed to be relubed with light oil. I kinda use the if it works do not mess with it approach.
There will be times that you just can not keep the ice off the guides and line. I do not have a cure for this except to dunk and shake. I wouldn’t use the tap the tip on the side of the boat method. This is a good way to break a tip or lose an eyelet. Seen this done more than once. If you are going to put the tip in your mouth to warm it up you had better look up river and make sure no one is hanging it over the edge of the boat.
Another thing make sure your batteries are at full charge, the cold temps really take a toll on half charged systems.
Good Luck
Dress warm
Have fun
Be Safe.