A method and apparatus for rapidly deballasting and lifting a substructure in a marine environment to a level where it will engage with a deck so that both act together as one body. Generally, the installation is carried out as follows. The substructure is towed to the mating site and upended (if necessary). The substructure may be connected to a temporary mooring and fixed ballast installed if necessary. Selected compartments in the substructure are filled with sea water until the substructure is submerged below the water line to a desired depth. A predetermined quantity of compressed air is pumped into lower tanks in the substructure. The deck to be joined with the substructure is positioned above and in alignment with the substructure. The proper valves are opened to allow the compressed air in the lower tanks to flow into the upper tanks that contain sea water. The compressed air displaces the sea water from the upper tanks. This increases the buoyancy of the substructure and causes it to move upward into contact with the deck. As the process continues, the deck and substructure will eventually act as one body.