Marine applications of aluminum are usually 6061. There is a place in Wicheetau (Wichita) called The Yard Store. It carries a lot of surplus from the aircraft manufacturing here (Boeing, Airbus, Cessna, Beechcraft, LeerJet and in the past Funk, Stearman, Travel Air and some others) thus being called the Air Capitol. Or at least it used to. Pickings are pretty slim these days at The Yard. But, I can usually find something there. Scrap metal prices being so high they are just scrapping everything instead of surplussing.
I have five technical plates, three of which are bare aluminum, a SS Freedom Plate and now two VDH plates. The two VDH plates, one is powder coated (wife) and one is bare (mine) and it shows very little if any corrosion. My Hammerhead plates are 6000 series aluminum I think and they also show no corrosion. I built a few prototype plates, since recycled, using 6061T6 and they did not corrode in any significant way.