An apparatus and method for the installation of a deck on an offshore substructure. The invention is particularly useful with a floating substructure. Two independent pontoons each have two columns spaced apart from each other that extend upwardly from the pontoons. On each pontoon, a support beam attached to the columns spans the space between the columns. In another embodiment, there is no support beam across the space between the columns. Each pontoon is provided with ballast tanks that allow the pontoons to be selectively ballasted or de-ballasted to control pontoon depth for receiving a deck or installing a deck on an offshore substructure. The pontoons may be ballasted down during transit of the deck such that the main body portion of the pontoons is below significant wave action and the columns present a relatively small water plane area. The pontoons allow the deck to be placed directly above the offshore substructure. For a floating substructure, the pontoons are ballasted while the floating substructure is simultaneously de-ballasted to transfer the deck to the floating substructure.