(FORTUNE Magazine) – When Microsoft co-founder Paul Allen pulled into Cannes in mid-May, people couldn't help but gawk at his newest toy. Completed late last year, the 414-foot-long Octopus is the largest private yacht in the world. While ten years ago a 200-foot craft was considered the height of conspicuous consumption, now corporate captains have set their sights much higher. Oracle CEO Larry Ellison, for instance, is expected to post a new record by launching a 460-foot boat this fall. "The only limit is the size of the bays of the shipyards of the world," says Diane Byrne, executive editor of Power & Motoryacht. "If someone can build a 500-footer someday, rest assured, someone will." With Byrne's help, we tracked down a few of the vessels moguls may be spotted cruising in this summer.