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. For soft growth you should have an additional biocide that help with the algae. Smart Solution is an excellent bottom paint for aluminum hulls. You can add Bioboost to the Smart Solution bottom paint which will further help with algae. I have listed links below to the products described above. These can be brushed, sprayed, or rolled. The Bioboost should simply be added to Smart Solution right before use. If you already have coating(s) on the hull, pressure wash, sand with 80 grit, and then apply Smart Solution.
How to Video: Aluminum Boat Bottom Paint Application