Steel underwater doesn't rust very quickly. It's the steel above the water (or near the waterline) that rusts. Rust requires oxygen after all.
One problem with unpainted aluminum boats is that the aluminum quickly acquires a layer of oxide. This oxide rubs off on your clothes, hands, and whatever else touches it. That's one of the reasons you see aluminum boats with unpainted topsides, but painted decks and cabins where people need to touch it. Aluminum is harder to keep paint on than steel.
One problem with unpainted aluminum boats is that the aluminum quickly acquires a layer of oxide. This oxide rubs off on your clothes, hands, and whatever else touches it. That's one of the reasons you see aluminum boats with unpainted topsides, but painted decks and cabins where people need to touch it. Aluminum is harder to keep paint on than steel.