A vessel includes an heavy loaded catamaran, one or several movable buoyant roll-on/roll-off platform(s) (3); a moving element is provided for draft variation in heavy deep seas so that the platform(s) (3) may be raised or lowered between the hulls thereby selectively changing the flotation of the vessel. In the upper position, the vessel is a deep-sea catamaran, in an intermediate position the platform(s) is buoyant and a canal is formed between platform bottom and catamaran keels, in the lower position, platform bottom and catamaran are even-keel converting the catamaran in a shallow water craft, in the lowest position, the catamaran is raised above water for docking.