From the Super Mario Wiki, the Mario encyclopedia
World 8-Ships is the second level of Dark Land in Super Mario Bros. 3. It becomes accessible after clearing World 8- and its own completion opens up the Hand Traps. This level is accessed automatically when the player is over the icon on the map.
This is a horizontally autoscrolling level. The player starts on the bow of a floating navy ship, which is armed with several cannons and manned by many Rocky Wrenches. The first ship also contains the only power-up in the stage, and if the player is already as Super Mario, they have to crouch and jump in order to collect the Fire Flower. The level consists of three similar looking ships, though the amount of cannons and Rocky Wrenches on them varies, which the third ship having the most. Through the Warp Pipe at the end of the third ship is a chamber containing Boom Boom, who opens the path forward and complete the level upon being defeated.
The player also has the option of swimming underneath the stage, though will be constantly pushed down by the ships, losing a life if they are pushed too far down.