The main feature seems to be the giant trees that serve as masts. In addition, the sides of the ship have many doors with ladders that lead to the water to aid the Utan Divers in their work. The vessel also had one large paddle, on either side of the ship, that holds the special chambers where the Utan Divers actually work.
The phrase "mori no hito" (森のヒト) written on the main sail in English characters, under Shojo's Jolly Roger, roughly translates as "man of the forest".