They really don't. Ships were a massive expense and quite prestigious and often supply limited. No dirty peasant is ever going to be able to purchase one. A rich merchant sure, some guy walking out the forest covered in blood with a bag full of gold, unlikely. You also need to remember that the act of a advy leaving his home realm results in a auto ban. Following that logic what do you think would happen to anyone that provided the means for them to leave their realm and continent?
With the support of a noble, I wouldn't say it's impossible. Many advies carry around recommendations to be lifted into nobility. The cream of nobility, no less. So I think buying a boat will hardly be a problem, if you have the money.
