Another option or pre polish method - get some aluminum cleaner - the kind they use on aluminum truck trailers. I know that ZEP makes it ( http://www.superkleendirect.com/search.aspx?find=Zep+a+lume
), but there may be others. It is a solution of Hydroflouric Acid. It is a spray on rinse off type deal. Follow directions to the letter!!! It will CLEAN
the Aluminum to like new shiny silver condition -not polished -but will get rid of the black stuff you are about to rub off if you polish it.
Try it, real easy, and will look like a NEW boat - clean, bright Aluminum. You may decide not to polish.