Make sure your sacrificial Zinc is the correct size for your boat and like most have suggested, rinse it off after use like you would any other boat.
In fresh water you don't have the electrolysis effect so no worries at all, our 10' jon boat is over 40 years old and looks it but she floats really good! Have had to re-rivet and seal the others over time but that is from loading, reloading, sliding down banks, back up them, hitting rocks, trees, and my personal favorite, snake shot from a revolver.
Apart from the fact that an offshore Aluminum boat is expensive since it is a custom project between buyer and builder, they are more efficient to operate than same sized composite hulls due to to weight savings and require little cosmetic maintenance unless painted as the alloy will form a protective oxidized layer.
Check out Bloody Decks, AluminumAlloyBoats.com, or hit the Yacht listings on the www for aluminum boats to check them out. The Coasties and Navy use a wide variety of alloy boats of all sizes but my faves are the White Water big block Jet Boats built by Hell's Canyon Marine, Rogue Jet Boatworks, North River/Almar, Bentz Boats, and these guys: