Raw water is the water that comes from outside your boat, fresh or salt. The "fresh" water is the mixture of antifreeze and water that flows through the engine cooling it and then is, in turn, cooled by the "raw" water in the heat exchanger. If you do not have a heat exchanger your engine is cooled directly by the raw water.
To winterize the engine fill the fuel tank with stabilized fuel. This is especially important with the alcohol in modern gasoline. Run the engine to make sure the stabilized gas is through the system, especially the carb or injectors.
Change the oil and filter.
For the cooling system you must replace the raw water with RV(non-toxic) antifreeze. I do this on the hard by running a piece of tubing from the intake side of the waterpump to a gallon of antifreeze. Start the engine and run it until you have run two gallons through the engine or heat exchanger and the muffler.(This should also run the stabilized fuel through the engine.)
Then, if you have a gas engine,(not diesel) remove the spark plugs and spray a little oil into each hole. Crank the engine a second or two to coat the cylinders with oil. Replace the spark plugs and you are done.