The Vanguard, the Blue Marlin and the rest of the Dockwise fleet are ships designed to carry other ships, drilling rigs or anything else too large or heavy to be easily transported on a conventional ship.

FORBES: Will the Vanguard Semi-Submersible Ultra Heavy Lift Ship Keep Dockwise Afloat?

That's very different from conventional submarines, which basically dive by changing the ballast of the ship to make it sink.

CNN: Winged luxury submarines 'fly' underwater