I found something to clean the outside of your boat that my wife suggested to me. They are Mr. Clean Magic Erasers. They look like a white sponge and all you do is dip it in water and scrub and it does the job. All the other cleaners for the scum line are toilet bowl cleaners and do a number on your paint and dull it. Depends on how big your boat is, but it will only take one or two, by the time your done they just fall apart.