Virtually all of the JA deep sea trades would benefit from being allowed to build overseas. There could be some new trades made economical as well, such as short sea container shipping on the east coast.
Attached is a graphic of what happens every day: US gasoline is exported to Canada and Canadian gasoline is exported to the US, i.e. foreign flag ships passing each other going in opposite directions carrying the same cargo. The reason for this is the high cost of shipbuilding in the US and Canada. Yes, US flag ships cost more to operate, but the deciding factor is the ship cost.
If foreign built ships are allowed into existing JA trades, owners of ships built in the US would have to be compensated. This has been done in other countries such as Sweden. That would be a one-time issue.
