Well........
I'm pretty sure it was less than a pound of plastic explosives used in the Lockerbie Pan Am bombing and the tests of the "shoe bomb" type device on a grounded and out of commission 747 in the "airplane graveyard" showed that only a few ounces could destroy an airplane. The thing is Airplanes in flight will rip themselves apart with very little loss of structural integrity due to the stresses involved, if it's in the right place. Also, regardless of the amount, it wasn't C4 that was used in Lockerbie, it was Semtex, so comparisons are going to be off in any event. However, plastique is plastique and they all go boom and should all be taken seriously.
A cruise ship is a different type of beast built in an entirely different manner. Though a pound of C4 ("enough to destroy a diesel locomotive," according to things I've seen) would damage a cruise ship and could, if it was placed in exactly the right place, do considerable, even fatal damage to a ship of this size, it's far more likely that any such device would be used to terrorize, not actually sink the ship. The scariest thing would be if such a device could be placed where it would start a serious fire. Fire is the biggest risk to a cruise ship under most circumstances.
I don't know if you remember the ship that had a serious fire in their laundry within sight of the port of Miami a few years ago. It was originally a relatively small fire but it ended up gutting about 10% of the ship and threatening the engine room (and lots more damage) before they could get it contained.
As I typed this the cable news folks have said that the "officials" are saying it was all about nothing (move along, move along, nothing to see here), even though they ended up blowing the thing up themselves with their own detonators.
In my mind this and the incident yesterday are clearly meant as tests of security at this port by the jihadists, to see what they can get away with. This is a bad sign.
My understanding reading this thread is that they did NOT blow up the "C4", but just a small deliberate detonation to open the "package" so they could check it better.
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.