Jeremy, the concern was thermal shock, i.e. very cold water coming into contact with a very hot boiler. I don't know of any instance where this happened with a Scotch Marine boiler, but if one happened to be weakend by the force of the explosion, well, who knows?
This whole discussion may be moot, considering the initial torpedo impact, whiplash to the hull, popping of rivets and forward momentum would probably have been sufficient to sink the ship. The secondary explosion could very well be overrated.
Not entirely moot. You really can't depend on what you read in a book- but go to the original sources. Should you look at original survivor letters, testimony, and what not- it can help you draw your own conclusions as to what may have happened. I suspect if people relied on original sources, not as much information would be out there today.