Welcome aboard. Interesting you got that large of chunks out of the manifold drain holes. A small amount of rust is normal. The plug on top appears to be a sensor and nothing for winterization. I didn't see any heater hoses in your pictures. Next year, I would recommend fogging the engine through the carburetor as it is easy to do. In the spring I would recommend using a carb cleaner as a good operating carb makes for a happy engine and operator. That year boat had two different type engines the Indmar and the Pleasurecraft which I have so when ordering parts remember to specify which type.