The ban on cabotage actually came in two parts; passenger traffic cabotage was barred under the Passenger Vessel Services Act of 1886, but cargo traffic cabotage was not barred until the much later Merchant Marine Act of 1920.Is the ordinance banning cabotage to no US owned and crewed vessels still in force? The one you mention must be its close kin.
The dredging act restrictions came about because of a rather bizarre conspiracy theory / mass hysteria in the wake of the 1900 Galveston hurricane that foreign dredging ships that came to assist with the cleanup were STEALING AMERICAN SOIL. This is why the Foreign Dredging Act, unlike the Passenger Vessel Services Act, has no exceptions for emergency situations.