It basically allows you to create a man-made oil well wherever water and power exists.
“Using renewable energy to convert the water would be great.”
But they’re not using renewable energy. And I think all they’re doing is generating electricity from steam turbines powered by the heat of the sub’s atomic reactors, using that electricity to power the hydrolysis of water into oxygen and hydrogen, somehow extracting dissolved carbon dioxide from the seawater, and then synthesizing hydrocarbons from the resulting molecular materials.
Quite frankly if they can do all of that for six bucks a gallon, I consider it a near miracle. Not to mention I’m skeptical that there’s that much dissolved CO2 in seawater. But, hey, what do I know?