"Some people think that you should not defend against underwater attacks until you've suffered an underwater attack."
Gotta love that "some people".
Strawman at its best!
"I wonder what these people were saying the week after 9/11, when everyone was saying we should have prevented that event, the first in history of its kind."
Why don't you find some of "these people" and ask them yourself?
Just about everyone else has heard of Kamikaze pilots. Unless you somehow did not learn about WW II in school?
"Maybe they are being consistent--but there were plenty of people who thought we should have been proactive rather than reactive."
In some cultures, beating a strawman is considered a sign of virility.
"I'm obviously in the minority here, at least among those who stick their necks out to comment. I'll quietly bow out now. Enjoy your nice homogeneous discussion."
Maybe if you focused on the specific points instead of kicking your strawman you'd make some progress?
No one is saying that terrorist divers with limpet mines could NOT happen.
What is being said is that spending money to prevent such an attack is a waste of money because the terrorists will focus on easier ways to strike at easier targets.
That is the essence of "movie-plot threats".
Look at what has to happen for your divers with limpet mines attack to work.
#1. Terrorists have to be in the country (or close enough to swim ashore).
#2. They have to have a specific kind of explosive device.
#3. They have to have diving gear.
#4. They have to be trained in the use of such gear.
#5. They have to be trained in the use of limpet mines.
#6. They have to use their diving skills with the diving gear to get to the ship to use their demolition skills to attach the limpet mines and damage the ship.
It makes a good movie script.
Or the terrorists can:
#1. Be in the country.
#2. Get a car bomb from someone with experience in making car bombs.
#3. Use their driving skills.
#4. To drive to the mall and blow themselves up along with a bunch of other people.
Fewer steps means better reliability. And less chance of detection. What's one more car at a mall? There are lots of cars there. What's one diver around a ship? That depends upon the ship.