To winterize a boat, fill up the fuel tank in the boat almost completely, and add some gasoline stabilizer to the fuel. Make sure that there is enough space left in the tank for the fuel to expand. Seal all the fuel valves using duct tape to prevent condensation from causing corrosion in the tank.
After sealing all the fuel valves, remove the engine flame arrestor, turn the boat's engine on, and spray the carburetor with some fogging or two-cycle oil. Shut off the fuel supply to let the engine get rid of excess fuel, turn the engine off, spray some oil inside the cylinders, and spin the engine several times to get the oil on the spark plugs. Reinstall the plugs without connecting the wires. Remove all the coolant from the engine block of the boat, and fill it up with antifreeze suitable for your particular boat.
Take the battery out of the boat, charge it completely, and place it in a safe, dry place to store during the winter. Check the battery several times during the winter to ensure that it is still charged and that the water level is not too low. Before putting the boat away for the winter, examine the vessel's propeller and hub to make sure that there are no dented or bowed blades or widespread wear. Repair any damaged parts you find before winter.