be patient.
polishing an aluminum boat is a fools errand unless you like re-polishing it often.
however if you are going to do it, here is how
you start at 240 grit, and sand the whole thing, you repeat with each 320, 400, 600, 800, 1000, etc. each unil you get to your 2000.
then you bring out the buffing rouge and wheels and you go over the whole boat doing a color buff. expect to go thru about 3# of rouge and about a dozen wheels
at that point, you removed all the oxides from the outside of it, and aluminum will soon corrode in the open air. do not touch it with your fingers, you will etch your fingerprints into the freshly polished surface. so you have to seal it with sharkhide or similar. once every few months, you will need to rebuff it, and re-apply the sharkhide
a cheaper and easier way is a vinyl wrap.
