You WANT closed-cell spray foam insulation to SEAL the aluminum hull. Besides, the close-cell foam provides considerable buoyancy. Calculate by multiplying the surface area of your hull by the average thickness of the spray foam times the density of FRESH water. Yes, the ocean is salt water, but the difference will account for at least some of the density of the expanded foam itself.

Skip the paints and coating, unless you find something that's more of a primer to help the spray foam stick even better to the hull while sealing the hull against corrosion even better than the spray foam.

If you have critical wires or other runs, lift them away from the hull then wrap key lengths of them in a plastic U such that you can spray insulation foam behind them while preserving them from contact with the foam while providing a narrow channel to be able to find them. If you need full-width access, jam some cardboard lengths into the U to maintain full width through the foam.

Various means can be used to gain very quick access to the hull through the foam, which is then easily repaired by simply spraying more foam.

Reading through the comments from consummate experts with many more years experience than I have merely confirms to me that closed-cell spray foam is definitely the way to go.

My experience: Building three sheds: Wood with fiberglass, steel with fiberglass, and steel with closed-cell spray foam.

Winner by a long shot, and for many reasons: closed-cell spray foam.

Downside: Carry a portable air device with full face mask in case of fire and you need to extricate someone from the cabin.