Come a major war, how many merchant ships will actually be willing to sail into our waters/ports? With a massive number of merchant ships being flagged in some rather dubious and questionable 'allied' countries, who's to say that these ships even sail close to our ports?
The top 10 flag states by registered tonnage are: Panama, Liberia, Marshall Islands, Hong Kong, Bahamas, Singapore, Greece, Malta, China and Cyprus.
I'd be willing to bet that ships from the Liberia, Marshall Islands, HK, Singapore, China, Cyprus and possibly Malta, all suddenly develop 'engine problems' or 'maintenance issues' and won't enter North American waters/ports.