A method for joining modules of ships. Ship modules are fabricated in individual bays of fabrication buildings and then moved out of the buildings for attachment together to form the ship on water. This eliminates the need for large dry docks or launch areas. The modules are floated and positioned such that the edges to be joined are adjacent or abutting each other. A flexible strap may be positioned tightly against the joint exterior to prevent water entry or a floating structure may be positioned under the modules such that a circular cross section backing seal the joint to prevent water entry. The interior of the modules is dewatered and the modules are then welded together. The modules may be positioned adjacent each other above the waterline through the use of a barge and dry dock to eliminate the need for dewatering.