Abstract: A floating unsinkable nautical craft comprises a hull having a keel portion below the water line and an upper portion. A longitudinally extending passenger compartment is provided at least mostly in the keel portion while a pilot's cockpit and passenger entrance are provided in the upper portion. Longitudinally extending dome-shaped observation windows are provided in the sides of the passenger compartment, the windows being of such length and height as to provide several passengers with an underwater view horizontally and downwards. A separate stabilization and bouyancy structure is connected rigidly to the upper portion of the hull and extends laterally to both sides with flotation and water ballast chambers at the ends. A pump is provided for varying the ballast to vary the draught of the craft.