If cargo is moved, you can expect longshoreman are usually (but not always) involved. The ILA represents longshormen on Atlantic and Gulf Coasts, Great Lakes, major U.S. rivers, Puerto Rico and Eastern Canada. The ILWU represents longshoremen on the West Coast of the United States, Hawaii, Guam (I believe) and in British Columbia, Canada.
The teamsters have gotten into the game and have their own longshoremen affiliates. But they are a fairly minor presence on the waterfront in comparison.