Semtex or C-4, the key ingredients are the same. Regardless, one pound of the stuff, concealed in a cassette player, blew up that plane.
Correct (which you are) or not, that wasn't my point. The tests done with considerably less plastic explosives in the shoe bomber investigation on a grounded 747 were sufficient to destroy that plane (blew off the tail section, as I recall) and that was without any of the stresses of flying at hundreds of miles an hour when the explosives went off.
A cruise ship is built to considerably different standards. A pound of C4 or Semtex would do considerable damage, but unless put in the right place or unless it set off a big fire it wouldn't be likely to destroy the cruise ship the way it did that 747 that came down in Lockerbie . It's several orders of magnitude more robust an engineering question based on what it is the craft is designed to do.
You make airplanes out of as thin and light a material as you can (usually aluminum at the upper end of the stress for the material) and cruise ships out of (relatively) thick steel with as little stress as you can get away with. You're talking completely different economic and engineering design criteria.
And that makes for a huge difference, kinetic energy wise.
I'm not disagreeing with your post. I'm trying to provide perspective to folks who picture a single pound of plastic explosives (of whatever type) taking a multi K Ton cruise ship and blowing it into itty bitty pieces in one shot. We're not talking a small nuke or antimatter here. It's a very powerful chemical explosive and, as I said, can "blow up" a diesel locomotive, which would ruin the whole day of a cruise liner. But a diesel locomotive (let alone a 747) is to a cruise ship as a hand grenade is to a small nuke (yes, that's way too extreme, but you get where I'm going).