An automatic boat washing system including several brush assemblies mounted on a vertically movable cage supported by a mainframe assembly and side ballast tanks is disclosed. The vertical position of the movable cage and brush assemblies may be adjusted in relation to the mainframe and ballast tanks during operation of the system. Rollers are used for supporting and drawing boats through the automatic boat washing assembly. The automatic boat washing assembly of the present invention may be used to clean powerboats in areas below the waterline in order to improve boat performance.