It should be a lot easier to move anything across the stage than to make moveable scenery, unless you have the luxury of a rear projection system. The ferry could be built on a bus -- a platform with casters. You can have just a simple flat cutout of the boat on the front, or construct an entire vessel on the platform. Casters should be fixed (not the swivel type) and lined up perfectly parallel for a bus that moves on a straight line back and forth. They should be large enough to support the weight of the bus and actors. If in doubt, use larger rather than smaller casters. You can pull the bus with a rope hidden behind simple cutouts of waves. Water can also be simulated by a blue or gray sheet of very lightweight fabric, stretched parallel to and a few inches higher than the stage floor and waves created by eigher shaking the ends of the fabric or blowing air from a QUIET fan between the fabric and the floor.