Louis I did it on an old aluminum I wanted to camo paint for duck hunting, pain in the ass, as Crowbart says you need to etch the aluminum to get the oxidation off (then quickly before our crappy air with the pollution starts to do it again you paint/spray a good aluminum primer on it, let that dry and then paint a good over coat color. Went through the instructions supplied by paint shop and painter.
2 years later it started peeling the paint off soooooooooooooooo, I would take the boat get some powdered cleanser like ajax wash and etch the aluminum good, make sure there is no gunk left on it and grab the good spray paint can and start (or your paint gun). biggest thing is where the water funs along the aluminum when boat is in the lake is where peeling will start (especially around the rivets) that area needs to be real clean.
good luck