An apparatus and a method for cleaning the hull of a vessel include a washing unit having a plurality of washing modules of the hull, suitable for washing the hull in a submerged configuration. Each washing module is equipped with mobile connectors for the mobile mechanical connection of one washing module to an adjacent washing module, to allow a reciprocal movement between the adjacent washing modules during washing and/or movement. Each washing module has washers of the hull suitable for scraping dirt from the hull. Each washing module is equipped with drivers to allow the positioning and movement of the washing module with respect to the hull and/or with respect to other washing modules during the cleaning of the hull.