What domestic shipping industry? Allow foreign lines to move cargo between the continental U.S. and Alaska, Hawaii, Puerto Rico, Guam, and you remove the need for a domestic shipping industry. And since foreign lines can move between U.S. ports on the coast then why not up and down rivers and lakes? If cost is your only consideration?
And why we're at it, why not allow foreign airlines to fly passengers between U.S. cities? Let Air France do the New York to Dulles run, Japan Air fly LA to Honolulu, Aer Lingus fly Boston to Chicago, Aeromexico fly Dallas to Miami, and so forth. Imagine the savings that could result from that kind of competition?
Of course the outcome of all that is you would lose just about your entire U.S. merchant marine, your domestic ship building industry, and probably a couple of airlines but hey, you're saving money.
Oh, I don't know ... maybe the one that operates on some of the busiest shipping lanes on the planet?
In a typical year, the volume of freight traffic on the U.S. domestic waterway system is something like two and a half times the volume of cargo that moves through the Panama Canal.