If the new build Jones Act short sea shipping ships were free, short sea shipping would still not be competitive with trucks due to the high cost longshoremen.
Tugs and barges are much cheaper than ships on short runs. They are also capable of serving many more small ports with minimal facilities and lower port costs.
US shipyards build good quality tugs and barges at competitive cost. They could build small ships competitively, if there was sufficient demand.
Jones Act ships are competitive with tug and barge for high value, time sensitive cargos, or on longer runs: Puerto Rico, Hawaii, Guam, Alaska.