Earlier quoted context omitted.
More realistically without the Jones act, ships wouldn't be built or operated by the US at all. International vendors can do this cheaper. You'd instead see all domestic shipping be entirely dependant on third-party international operators paying third-world wages to third-world crews, and you'd have next to zero recourse against them if they, say, run one of their ships into a bridge, or spill a few million litres o…
They already are not built in the US at all. This is already true today. We already build less than one oceangoing Jones Act compliant ship a year. The US shipbuilding industry can hardly get any worse than it already is today.
There are a few good reasons to repeal the Jones Act (reduce shipping and trade costs in Hawaii, Alaska, and Puerto Rico) and a lot of really bad ones (the domestic shipping industry will be completely killed, and you're inviting unbounded liability from unregulated, fly-by-night international actors who don't give two craps about our laws.)
The way ocean shipping currently works is entirely incompatible with any national rule of law. Flags of convenience and corporations with non-existent liability mean that nobody in the international industry is actually following any of the rules.
The domestic industry has to follow them, which is the reason why it's not cost competitive.