No, not “opening up to foreign manning agents”.
“Foreigners” doesn’t mean “uneducated villagers at $100/mth.”, but to open up to qualifies foreign crews that hold the necessary “green card”, or whatever form of “work permit” may be required.
If you had a larger fleet of ships in overseas trade there would be more jobs to fill, thus more opportunities for employment. Domestic trade would still be reserved for US citizens.
If you are worried about recruitment to Officer position that is legitimate, unless the system is changed to where there are not just military run Academies, but private or public Maritime Schools that teach full courses, leading up to sitting for exams per STCW requirements.
A system of “apprenticeships” or “cadet” time to get the necessary practical experience and seatime to obtain entry level CoC. Thereafter seatime at Officer level and additional schooling as required to upgrade to higher ranks, replacing the piecemeal method of taken one course here and another there, to obtain the right to “test” for a hundred different levels of licenses, limited in tonnage, area and trade.
All special training and certification required for individual types of vessels or whatever could still be offered by private Training institutes approved by the Maritime Authorities of the land.
Simply put, adopt STCW Convention in full for anything that move outside the baseline and thus American mariners would be able to work on ship under any flag in any waters.
If this sounds like some sort of “utopia” it isn’t. It is how most countries in the world teach, train and certify their Mariners.