a boat is only good if it is huge, like some kind of freighter. zombies are still mobile in the water and could probably get into a small skiff or row boat.
you'd need to have something that is high off the water line, so no one could simply climb aboard - living or dead
and then you have to make sure that you have all the necessary tools or skills for water making as well as fishing supplies etc.
and finally, you have to worry about defending against pirates.
the best boat scenario would to be to hijack a large sailboat with a pre-selected crew and captain, fortified with supplies, weapons and one damn good plan. not as high off the water as a cruise ship, but you'd never have to worry about running out of fuel