For the record, in my mind, the weapon's officer only fired a single torpedo at the megalodon, since he probably decided one torpedo would probably be sufficient in driving most predators off (those torpedoes did lay waste to an entire fleet). What blew the sovereign up was that the torpedo tubes were close enough together that they set off a chain reaction when one of them exploded inside the tubes.
EDIT: Bite force of Megalodon: Over 40,000 pounds per square inch, or about 276 MegaPascals)