While it's true the Coast Guard escorts the ships into and out of the ports, and scuba divers are under the ships when they are docked, it is also true that once the ship is out of territorial U.S. waters it is on it's own.
I've brought this subject up a couple of years ago and boy did I get slammed for even mentioning such a possibility. "The bags are all screened, therefore it's safe." "Port security is very good, therefore it's safe." "The security on the boat is very good, therefore it's safe." are some of the comments I got.
But to be sure, it would be simple for a 40 ft. Scarab type boat loaded with explosives to run into it 100 miles off shore.
I am also concerned about the safety of the cruiseships.