Winterize a boat by running the inboard engine to warm it, and change the oil while the engine is warm, which allows the oil to more fully drain. Use the flushing port to provide cooling water. Remove and dispose of the oil filter before refilling engine.
Check the manufacturer's suggestions for the fuel. Some manufacturers advise both adding stabilizer and filling the tank because there is less room for condensation inside a full tank. Other manufacturers believe the risk of ethanol spoiling during the winter is too great and advise only adding stabilizer. Do not forget to change fuel filters and water separators.
Check your stern drive, and remove any barnacles or plant life from the lower unit. When draining the gear case, inspect for moisture within the oil. If you see any, repair any leaking seals before taking the boat out again in the spring. Next, use soap and water to cleanse the lower unit, and check for pinholes or cracks in the rubber boot of the stern drive if the boat has one. Next, grease all of the fittings, and inspect the fluid levels in the lift pumps or the hydraulic steering. Finally, examine the owner's manual for additional recommendations for winter.