If an Al boat puzzles you an Aircraft carrier must be simply amazing.
when i was in school, i remember these civil engineering departments like to construct, as a demonstration, a concrete "canoe" and paddle that in a lake or river. not the most efficient watercraft, but it floats.
yes actually the overall density of an object is related to center of gravity and composition of the boat, since the boat is not the Al. part only but also the air inside the total volume, this what makes it lighter than water