There are lots of different aluminum alloys. Some have good corrosion resistance and some don't. If the right alloy is used and care is taken to avoid galvanic corrosion, then aluminum is a very good material for boat building. As noted above many large yachts are built in aluminum. There are several aluminum boats locally that have sailed extensively for years without problems. For example Palawan VI is at the marina in Southwest Harbor right now. It is aluminum, 34 years old and in as new condition. Another aluminum yacht that summers here is the 140' ketch Rebecca. I also have friends who have a 40 aluminum boat named Grendel that has sailed the north Atlantic for years with no corrosion problems despite not being painted above the waterline.
As far as weight goes, an aluminum boat will generally be lighter than fiberglass for equal strength in contrast to steel (particularly for boats under 50').
As far as weight goes, an aluminum boat will generally be lighter than fiberglass for equal strength in contrast to steel (particularly for boats under 50').