Revealing mistake: When one of the bad guys is being dragged around by the monsters and thrown against pipes and walls and such, in wide shots you can see there is no monster really pulling him, and from the stiffness and position of his body, he is likely on some sort of device to swing him around.
Revealing mistake: Occasionally, when the characters are being eaten by the sea-monsters, you can see pieces of the computer-generated beasts going "through" the victim.
Revealing mistake: When Treat Williams and Famke Janssen are escaping on the jet ski look closely. You can see it's a male stunt double for Famke due to the nose size.
Answer: According to some good old Google Fu, a single torpedo hit could absolutely destroy and sink a cruise ship since they're not as fortified as a warship would be. They're meant to carry passengers, not go to war. Granted, it probably wouldn't be quite as dramatic an explosion, but you could also argue that any other explosives on Finnegan's ship, plus the cruise ship's fuel, could have been ignited in the blast. You also have to factor in that the ship was already heavily damaged from the monsters attacking and was in a more fragile state than it would otherwise be.
TedStixon