Cheated a bit with my big boat and built it mostly on land, digging it out. Going to dig out this section of land anyway and make a proper ocean channel.
This ship has two lower decks, one of which is underwater, so it sits four blocks beneath the surface and six above, plus the fore and aft sections. It is eleven blocks wide at the center and roughly forty blocks long. It also gobbled up about five stacks of wool, so yay, all that sheep abuse was useful!