Safeguarding your boat over the winter cam mean various things. One is preventing theft of valuable items.
According to the Boat Owners Association of the United States (BoatUS), the three items most commonly stolen from boats over winter are electronics, outboard motors, and outdrives.
Of course the best way to prevent electronics or outboard motors from being stolen is to remove them and store them at home. For anything that is permanently mounted, photos should be taken and serial numbers written down, just as you should do with items in your home. This will help recovery, although I would not count on that.
In the case of larger outboard motors, secure them with a good lock. The same goes for an outdrive, if it cannot be removed and stored at home.
Another line of thinking for safeguarding your boat over winter is performing the essential preventative maintenance operations.
Fill the fuel tanks and add stabilizer.
Flush the cooling system and drain all water from inboard and I/O engines.
Fog the engine to prevent rust.
Replace the oil from inboard and I/O engines.
Change the lower unit gear case lubricant on outboard and I/O engines.
Store outboard engines in an upright position.
Protect electric terminals by spraying with moisture-displacing lubricant.
Drain water from the bilge and keep the drain plug out.
Life jackets should be removed from the boat and stored where they can air out.
Keeping a maintenance chart will help you to remember which maintenance functions have been done, and which need to be done. Some of the things which boaters may do either before or after winter storage are replacing the oil and oil filter from inboard and I/O engines, checking the propeller for nicks which can reduce performance and fuel economy, inspecting steering systems, cleaning the backfire flame arrester of inboard and I/O engines, greasing or replacing wheel bearings, and checking the electrical system.
Then there is the matter of 'green' boat storage. The term 'green', which I dislike more and more every year, is generally understood to mean environmentally friendly. BoatUS offers these suggestions for green winter boat storage.
If you add antifreeze be sure it is a safe, non-toxic marine antifreeze that contains propylene glycol. Usually it is pink but it may be other colors. Never use automotive antifreeze which is toxic and may cause illness if used in freshwater systems.
Collect and save all oils that are removed. Usually they can be put in one container. Take them to the recycling station of your marina or a local gas station.
Wash a boat before putting it into winter storage. While in use through summer, a boat should be rinsed regularly with fresh water to prevent the buildup of dirt. Dirt which remains on a boat too long gets harder to remove and it may cause stains.
Use an environmentally friendly boat soap. BoatUS tested 20 boat soaps to determine whether they really are environmentally friendly, and whether they do a good job of cleaning a boat. The least toxic boat soaps were 3M Boat Soap, Meguiar's Boat Wash Gel, and Thetford Boat Wash. However, 3M Boat Soap did not break down quickly. Soaps which break down quickly have less impact on aquatic life. Meguiar's Boat Wash Gel was not a good cleaner.
BoatUS rated Thetford Boat Wash as their first choice because it ranked excellent for toxicity, biodegradability, and cleaning performance.
If you do not put your boat into winter storage because, like me, you are always hoping the weather will be tolerable enough to do some winter walleye fishing, remember that the same cautions that apply to boats in storage apply to boats that are kept at the ready, with the possible exception of stabilizing fuel.
Perhaps the most important thing to remember for preventing winter maintenance is unplugging trailer lights before launching or loading. I have learned through decades of frustration that all electrical connections should be sealed with lubricant designed for the purpose, then except for the plug between the trailer and vehicle, further sealed with a waterproof material. Anything to avoid working on the boat or trailer during winter.