Without the Jones Act, most of the tugs, barges, OSVs and fishing vessels would be foreign built.
Without the Jones Act (Magnussen Act too) many of the tugs, barges, OSVs, and larger fishing vessels would be foreign flag with foreign crews.
The last count that I saw indicated that there are only around 100 deep draft foreign going US flag ships that employ about 6000 crewmen, assuming two crews per ship.
As I recall the count on Jones Act ships was something like 70 ships probably employing something like 3000 men, assuming two crews.
About 10,000 tugs probably employ around 100,000 men, assuming two crews
There must be at least 100,000 fishing vessels.
I don’t know how many OSVs, ferries, small passenger, and misc vessels there are, but its got to be thousands.
The last figure that I saw indicated that there are somewhat over 50,000 mariners with an MMC.
For many tugs, and most small passenger vessels, only the master and mate are required to have a MMC. Most fishing vessels do not require any MMC.
The Jones Act does a lot more than protect 70 ships and 3-5 shipyards.