A vessel designed to operate efficiently as both a catamaran and air cushion vessel can travel at low speed in a catamaran or displacement mode, high speed in an air cushion mode, and amphibiously with the aid of deployable side seals. The vessel includes catamaran hulls and flexible air cushion bow and stern seal systems together with retractable side seals having hinged doors containing inflatable, retractable flexible components. The doors form part of the side seal system in the deployed position, and protect the flexible components of the seal in the retracted position. The arrangement reduces the drag associated with the side seals while the vessel operates in its catamaran or surface effect ship modes, and prevents damage to the seals during operation in the high speeds SES mode.