If you do not have any coatings at all on the bottom of the boat, we suggest to sand the hull with 80 grit and then apply Tuff Stuff High Build Epoxy. It is a water tight barrier coat designed for use under the waterline and with aluminum hulls. Use 2 coats. You can apply the 2nd coat within a couple of hours of the first coat. (up to 6 days)
After you apply Tuff Stuff marine epoxy primer on hull, be sure to use an aluminum safe bottom paint. The bottom paint needs to be applied the same day as the Tuff Stuff marine epoxy primer. Bottom paints in general are designed to repel hard growth. Colorkote is an excellent triple biocide bottom paint for aluminum hulls when in salt water. We recommend Mission Bay for fresh water. I have listed links below to the products described above. These can be brushed, sprayed, or rolled. If you already have coating(s) on the hull, pressure wash, sand with 80 grit, and then apply your paint choice.
How to Video: Aluminum Boat Bottom Paint Application
See here for all aluminum safe bottom paints.