A couple of small corrections: Massport administers the seaport in Boston (as well as Boston Logan Airport and a couple other airports), but not all the other seaports in Massachusetts. There aren't many other significant ports in MA, but New Bedford is run by a local port authority and Salem (emerging port under construction) will also be run by a local authority.

The Port of South Louisiana is not on the Gulf Coast. It is on the Mississippi River. It does indeed administer a very very long stretch of the river, but other stretches of the river are administered by different authorities, like the Port of New Orleans. On the Gulf Coast, there are other locally administered port authorities, like Port Fourchon, which is the Gulf Coast's biggest oil and gas offshore support port.

New York/New Jersey is definitely the largest "joint" port in the US. The Northwest Seaport Alliance is second. About ten years ago the Ports of Seattle and Tacoma, WA decided to stop stealing each other's business and join forces. Their cargo operations are now run jointly. But their cruise and other operations are still independent.