Yes, you are correct.
I don't think that most have any idea of how many containers of goods are shipped into the U.S.
Or what we will need to survive, if the incoming containers are stopped.
Everything we eat, wear or use, has been in a container.
Remember the longshoremen's strike a couple years ago? The ships started backing up at sea. Retailer's shelves sported apology notes. No merchandise to sell. We are critically dependent of smooth operation of seaports, trucks and rail transport to move Asian produced goods to our markets. A simple attack on the west coast ports would have a severe effect. Direct attacks against the container vessels would place the safety of that means of transport in question.
From what I understand, there are ~2500 containers on most of the modern cargo ships. I have no idea how many ships are unloaded in our ports within a 24 hour day though.