Paint may not be necessary in fresh water, but in salt water, I think it's necessary, to seal the aluminum from corrosion. ESPECIALLY if it's below the waterline. If you don't paint anything else, you should at least paint anything below the waterline, using zinc chromate, then a couple of coats of interprotect, then your anti-fouling paint.
While it's true that aluminum forms an oxide almost immediately, this oxide can continue to form until the aluminum looks like alka-seltzer, and at that point, it's screwed.
Remember that when painting aluminum, you need to etch it with phosphoric acid, followed by a good rinse. Then prime it with zinc chromate primer. You absolutely must use the zinc chromate primer, if you use other types of primers, you won't get good adhesion, the paint will almost always flake off.