Well, I wasn't of course there but I'm pretty certain we would be dealing with an implosion rather than an explosion, followed by a fireball. The sparks visible from outside could be the "remnants" of the fireball that erupted below.
We do have eye-witness accounts of later steam shipwrecks that describe exactly such a sequence of events when cold water floods the boilers. I'll post a link to a part of an interview with a survivor of the SS Carl D.Bradley, where he describes this during the sinking of his ship:
When the throttles are suddenly shut for a coal-fired steam boiler, there will be upset conditions that result in unburned fuel coming into the stack. That could well account for the visible sparks, etc.