(Article I, Section 8)The Congress shall have power...(t)o regulate commerce with foreign nations, and among the several states, and with the Indian tribes....
(perhaps the most-abused and -misunderstood clause of the Constitution)(t)o make all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this Constitution in the government of the United States, or in any department or officer thereof.
The line of reasoning is that since virtually all of the commerce that takes place at a dock is with a foreign nation, Congress would have a role in this. The scope of this role would then fall into debate (IMHO, the further the claws of government remain from this, the better).