Other wonderful possible ship bombs include bulk carriers loaded with fertilizer. The Texas City disaster was an explosion on the same scale as a small nuke. Likewise in WWI an ammunition ship detonated in a Canadian port it was in the same order of magnitude of explosive power as the Hiroshima blast. Nor do the Tangos need to restrict themselves to the most conventional explosives. Bulk Carriers loaded with sugar and dosed with a few tons of a particular chemical would take out a large portion of a city. Wheat carriers have a whole lot of grain dust and are a major disaster when they burn. Chemical and fuelk barges and tankers also have a major destructive potential.
The USCG was overstrectched in 1999 and things have just gotten worse since then.
Stay well - stay safe - stay armed - Yorktown
I think that the Sea Marshal program in effect today is screening potential dangerous cargo ship bombs very closely, it's possible but would take a major terrorist team to hijack a HAZMAT ship and then kill the Sea Marshals etc. Not that it's impossible, but it would most likely be a one-time event. (Of course one time is plenty if the result is worse than 9-11.)
But a dozen or a hundred ANFO bomb containers could be set out on the worlds oceans with GPS triggers, and it would take stopping world trade utterly to track them down before they exploded in our cities while being delivered.