Now that is a damn valuable link!
I intend to look into this sort of thing a lot more for my future travels. Just being able to give the middle finger to luggage overages & have a bed to sleep on instead of an airport bench during a 6 hr layover from midnight till 6am might make all the difference. And I wont have to turn off my ebook reader when the ship starts moving!
According to that page, ships must hire a doctor if they have more than 12 passengers, so it's just not worth it for most of them to carry more.