An aircraft having an airframe that extends longitudinally along an anteroposterior plane. A buoyancy system is provided with at least one pair of two floats arranged transversely on opposite sides of said anteroposterior plane. Each float is movable in elevation from a bottom position to a top position. For each float, the buoyancy system includes a blocking system for blocking each float by default in the bottom position, and for acting under a predetermined condition to block the floats present on one of the sides only of the aircraft in the top position.