One possible alternate way to get a lot of shipbuilding capacity on demand is to pay major overseas shipbuilders in allied countries to (a) build ships for within-US shipping and (b) devote some dedicated percentage of their production to the US in case of geopolitical emergency. The world's second- and third-biggest shipbuilders are Mitsubishi and Hyundai, both in closely-allied countries; surely there's some amount of money that would make them happy to help out. They're very good at making ships efficiently, much better than the US at this point.