From Dept of Homeland Security: The Maritime Infrastructure Recovery Plan

Found here. From the introduction:

The Maritime Infrastructure Recovery Plan (MIRP) (PDF, 63 pages - 1.8 MB) is one of eight plans supporting the National Strategy for Maritime Security. It was developed in collaboration with public- and private-sector stakeholders, as directed by National Security Presidential Directive-41/Homeland Security Presidential Directive-13. Its development was also coordinated with other supporting plans, especially the Maritime Transportation System Security Recommendations and the Maritime Commerce Security Plan because of their importance to the secure flow of commerce.

What the MIRP is:

-The MIRP is intended to protect the American economy by facilitating the restoration of passenger and cargo flow, specifically container cargo, in the event of an attack or similarly disruptive event. Container cargo is more likely to hold perishable items in immediate need of unloading, or items that are key components in the production of consumer goods.
-The MIRP includes an exercise plan to maintain a level of preparedness within maritime community. This plan recommends periodic table-top and field exercises, which align with existing related plans such as the National Response Plan and the Top Official program.

What the MIRP is not:

-The MIRP does not address long-term interruptions for conveyances that carry primarily non-perishable cargo. In addition, certain commodities, such as liquefied natural gas and oil offer very limited options for cargo diversion, as there are just four LNG ports, and oil refineries are already operating at 97 percent capacity.
-The MIRP is not a plan for the physical recovery of a port that has been impacted by a natural or man-made incident. Rather, the MIRP protects the economy by providing guidance for redirecting container cargo traffic away from the impacted port to an appropriate alternate port.

Also in the Maritime Security vein, a Heritage Foundation report on "Trade Security at Sea: Setting National Priorities for Safeguarding America's Economic Lifeline." From the Heritage website, some analysis of why it's important to the US to have good maritime security:

The Importance of Maritime Security. The importance of the maritime domain cannot be overestimated. Almost one-third of U.S. gross domestic product (GDP) is derived from trade, and most of America'’s overseas trade is transported by ship. According to the American Association of Port Authorities, $1.3 billion worth of U.S. goods moves in and out of U.S. ports every day. In addition, many major urban centers (more than half of the U.S. population) and significant critical infra structure are in proximity to U.S. ports or are accessible by waterways.

Maritime security also has a critical defense dimension. The vast majority of U.S. military forces and supplies sent overseas transit through U.S. ports. For example, in fiscal year (FY) 2003, the U.S. Military Traffic Management Command (now called the Strategic Distribution and Deployment Command) shipped more than 1.6 million tons of cargo in support of Operation Iraqi Freedom. America cannot be prosperous or safe without access to the sea.

During the next 20 years, maritime commerce will likely become an even larger and more important component of the global economy. The main elements of this transformation will probably include continued growth in the seaborne shipment of energy products, further adoption of containerized shipping, and the continued rise of megaports as commercial hubs for transshipment and deliveries.

Barring substantial and unanticipated reductions in the cost of air transport, this trend will persist for the next few decades. Over 10 million containers, which account for 90 percent of goods transported across the seas, entered the United States in 2005. The number of containers entering the U.S. by sea could double by 2010.

Seaborne transport will remain critical to defense as well. Despite the anticipated development of a new generation of long-range global strike aircraft and rapidly deployable future Army combat forces, it is highly unlikely that the U.S. military will be able to sustain a major campaign in the foreseeable future without the capacity to transport significant assets from the continental United States by ship.

UPDATE:
But a really good part of the Heritage report deals with the suggestion that the US focus on inspecting every single container entering this country (Was that Sen. Schumer?):

Misguided Port Grants and Inspections. To counter the “nuke-in-a-box” threat, some propose spending billions of dollars on container and port security. This argument fails on five counts:

The nuke-in-a-box is an unlikely terrorist tactic. If an enemy wanted to smuggle a bomb into the United States, an oil or chemical tanker, roll-on/roll-off car carrier, grain or other bulk vessel, or even private watercraft would be a more logical and secure way to transport it, either directly to the target (e.g., a port) or indirectly by landing it in Mexico, Canada, or the Caribbean and then moving it across a remote section of the U.S. border. Indeed, logic suggests (and most experts believe) that a port is more likely to be attacked from land than from sea, especially given the lack of visibility into the domestic trade network, the lack of protection on the landward side, and the ease of constructing explosive devices with domestic resources. Terrorists are more likely to construct smaller items (e.g., biological agents) domestically and then to deliver them through FedEx.
While nuclear smuggling is possible, so are dozens of other attack scenarios. Overinvesting in countering one tactic when terrorists could easily employ another is dangerously myopic.
Spending billions of dollars and deploying thousands of personnel to search every container and harden every port is an extremely inefficient and expensive way to stop terrorists from using cargo containers, especially when they would probably use other means.
There is no apparent viable business case for many of the proposed solutions for “hardening” shipping containers, conducting 100 percent physical container inspections, or requiring expensive tracking or monitoring devices. These measures would provide only minimal utility at the cost of billions of dollars in new duties, taxes, and operating costs.
Such efforts would divert resources from solutions that would measurably strengthen mari time security, including watching the back door of American ports through which trucks, trains, and barges travel daily.
As a matter of common sense, the United States should not attempt to make every cargo container and port into a miniature Fort Knox. Securing trade requires an approach that is more comprehensive and effective than just putting up fences and gates, posting guards at ports, deploying radiation detectors at every entry, and inspecting all cargo containers as they enter the country—approaches that would waste security resources by inspecting things that are unlikely security risks and create isolated, easily bypassed chokepoints to address specific (and unlikely) threats.

The better answer to the nuke-in-a-box scenario as well as the much more credible threats is to increase U.S. efforts to interdict potential dangers before they reach the ports and use the best and broadest possible intelligence available, generated from a combination of commercial information and intelligence. In that regard, security measures should focus on building capabilities that will address a broad range of dangers rather than fixating on a few Tom Clancy–like scenarios.