Boat winterizing begins with preventing any liquid, Fresh or Salt, water freeze. Let's put it in a basic list:
Fresh water Systems - includes your water tank(s) that go to your Galley, Head(s), holding tanks, A/C system, Deck wash down. (you may others)
Engines/Generators - includes their freshwater cooling system, PLUS their Saltwater System.
We can now do boat winterizing section by section. Get your Head hold tanks emptied and also empty your freshwater tanks. Some freshwater tanks have drains or you can disconnect the outlet hose on the bottom of the tank. If you can't get at the drain/hose or it does not have one, run your freshwater pump until the tank(s) are empty. In this way the tank empties and you can add nontoxic antifreeze that you can find in your marine store. The excess antifreeze will run to your bilge, where you already removed the water from. You want some antifreeze in the bilge incase rain water accumulates there during winter storage. At your freshwater pump that delivers water to the galley sink, head and shower, and deck wash down, etc, use the hose inlet hose there if you can or disconnect that one and use a temporary hose in the mean time. Have one end attached to the pump (inlet side), the other end put in the container of the nontoxic antifreeze. Marine stores sell a RV Antifreeze made for all freshwater systems, and is good to minus 50 below. With all your freshwater outlets, sinks, etc, shut off, turn on your pump. It will cycle like it usually does till it sucks up the antifreeze. Now turn on the furthest outlet first till the water coming out turns the color of your nontoxic antifreeze…normally red, and shut it off. Do this with the hot and the cold in the sink and shower. Do the next closer one and so on. Where ever your fresh water outlets are, remember your cockpit shower. Your water inlets to the heads have to have the antifreeze solution pumped thru, ending up in the holding tank, that's why we emptied it. So now your Heads, Galley, Deck, areas are done, including your bilge. Anything that water comes out of should be the color of the antifreeze. Do you're A/C systems the same way by find the pump and putting the inlet hose to the pump in the antifreeze solution. Run the A/C till the red comes out on all units.
When boat winterizing, your engines may have freshwater cooled engine(s). Here you will have to check the protection the freshwater is good to. This is usually a combination of freshwater and regular antifreeze. A tester can be found in any marine store or an auto parts store and are inexpensive. Check to make sure your antifreeze protection is where it needs to be in your particular area. Your engine(s) have a saltwater system also, which has to winterized. Take the inlet hose to the saltwater pump on your engine and put it in a container with the nontoxic antifreeze. Start your engine and see that the antifreeze gets pumped thru and out the exhaust. Do your other engine and/or generator the same way. Some like to change the oil and filter before they winterize the boat. Close all your seacocks and use any precaution that you need to in your area.
Taking the mystery out of winterizing a boat is basically what this is for. Always take any precautions you should. This is a generalization and should only be used as a reference along with other instruction that came with your boat.
Find a dealer or company in your area for boat winterizing or use the Categories in Marine Marketplace to view more of what you want.