A bit more info would help here. For instance:
Did the engine stop suddenly, or did it continue to run whilst making the noise
Can you turn the engine over, either with the starter or by hand?
What sort on noise was it? a single
big bang, multiple bangs, a screechy sort of noise or what?
Have you had a look at the oil
? Is it full of metallic flecks?
In general, if you have "thrown a rod" the engine will be either seized or only rotate through a partial revolution. There may be a hole in the pan or the side of the block. If you have dropped a valve into a cylinder, a look under the rocker arm cover will show said valve to be missing or jammed in an open position. If you have run a crank bearing, the engine should still be able to turn over, but with excess friction.
I've never blown up a marine
diesel, but lots of sad experience with racecar engines. When something severe happened to those (all V-8 petrol engines) it was readily apparent to the naked eye. The slower speed of the marine
engine may help disguise the damage, but some symptoms will surely be present!
Good hunting, and lets hope for some good news.