Once you let black streaks sit for awhile, they become entrenched. They come from dust and dirt that accumulates on the boat; add some rain and you''ve got black streaks down the side. The only thing you can do is at least one a week give the boat a good washing and add a bit of liquid car wax in the wash water. If you are at a mooring I know this is not a great solution but if you keep putting sandpaper to the gelcoat, one of these days you''ll have no gelcoat left. Better to wash and if that fails, use fiberglass rubbing compound (for boats, not cars), rub out the streaks and top with liquid or paste wax.
