Abstract: A semi-submersible marine craft for operation on the surface or in a semi-submerged condition has a pair of spaced parallel tubular water ballast floats disposed horizontally on either side of a vertical middle plane. The floats are disposed symmetrically relative to a central nacelle incorporating the power unit for propelling the craft and comprising a watertight cabin for the passengers. Control apparatus are provided for submerging and surfacing the craft, and the necessary control and safety apparatus for operating the craft. The floats and the nacelle have a frame structure made of an assembly of stainless steel sections which are covered by a skin of synthetic material. The nacelle has a cabin with a transparent semi-spherical front portion and side windows. On top of the cabin is hingedly mounted a dome for entry into the craft. Releasable ballast are disposed under the nacelle.