I've lived on the salt marsh for almost 30 years, and for most of that time, I have kept aluminum boats tied up at the dock. For the past several years, my boat has been at the dock for at least 300 days out of any year.
Here are some of my observations and tips.....
With welded bottom boats, if you notice, the center strake is welded down both sides. This is because the boat hull is stamped in 2 halves, and this center strake ties them together. Unfortunately, this much heat being put into that center strake makes it the most susceptible part of the boat for corrosion, particularly at the front end, where new bare aluminum is constantly being exposed from dragging the boat onto the beach, oyster rocks, etc.
After bottom painting your boat, you should install a keel-guard on your center strake, to prevent this problem. Because even with good bottom paint, if you drag the boat onto the beach, the paint WILL come off the center strake, and as soon as bare metal is exposed, electrolysis begins.
Any marine growth that attaches to an aluminum boat also makes it more susceptible to corrosion, as this growth etches itself into the surface of the aluminum, and unless you sand it off completely after scraping, it gives new growth a place to attach, as well as allowing electrolysis to etch further into the metal.
The FIRST thing you need to do is paint the bottom with cuprous thiocyanate-based paint...NOT copper oxide, this will destroy your hull.
Interlux makes the paint you need, it's called "Trilux 33" and it is specifically formulated for aluminum. It costs more than the cuprous oxide paint, but this is one place you don't want to be cheap.
Before painting with bottom paint, you also must apply at least 2 coats of Interlux "Interprotect" primer, or your bottom paint will fail to properly adhere, and will come off, leaving the boat with no protection. This is a 2-part epoxy, and it comes by the gallon. Again, it ain't cheap, but neither is having your boat welded to fix corrosion holes, or worse, having to scrap the hull because it's FUBAR from corrosion.
Now, as far as anodes, for a 16 foot johnboat with a 48 inch beam, you need about 32 square inches of anode for proper galvanic protection. On my boat, I have 2 thru-bolts going from the inside of the transom, with washers and lock nuts on the outside, and 5200 applied to them.
The bolts extend out of the transom by a couple of inches, so that the anode mounting holes align with the bolts, and the anode is secured with washers, lock nuts, and a backer plate. The backer plate is a wide strip of aluminum and it serves to keep the anode in place, even if the mount holes of the zinc erode away to the size of the washers and nuts. Without the backer plate, if the holes corrode away (and they will, as the bolts are SS) the zinc can fall off, leaving the boat with no protection.
You should also install a bonding wire to the anode of your boat. You can tie it in to the bonding wire used on the zinc used on your power trim, or, bring it into the boat, and connect it to the negative terminal of the battery. The bonding wire can be made by using some 1/16" diameter stranded stainless wire rope, with ring terminals crimped to either end (since it's stainless, you can't solder it to the ring terminals)
This one goes without saying, but, if you plan on leaving your boat at the dock, invest in a bilge pump and float switch, or one of the self-contained bilge pumps.....but remember, NEVER depend on a bilge pump to do its job. If there are heavy storms, you'd better check in between squalls of rain to make sure the pump is doing its job, or better yet, haul the boat out.
And on a final note, speaking of hauling out, every 3-4 months, you should haul the boat out of the water, to inspect the bottom for growth, change lower unit lube, grease the fittings, flush the cooling system of sand and salt, inspect the anodes, remove the engine cowling and give the engine a good rinse with fresh water, let it dry, then re-fog everything with some sea foam, hilco lube, or Boeshield.
Matter of fact, I hauled my boat out today for its quarterly inspection, as well as getting ready to do some modifications.