This doesn't make sense. Correct me where my understanding goes astray..
You were running fine for 30 minutes. Did you have eyes on the temp gauges during the time? That would be a good confirmation of whether things were working correctly before hand.
Then you ran over the shoal. The stbd engine stalled while crossing the shoal or shortly after. The port engine continued to run. How long was the transit over the shoal? It sounds like it was pretty quick? Seconds? More than a minute?
What makes no sense is for an engine to overheat and seize in such a short time. Even with a complete loss of raw water flow the engine would run for a while before overheating. The exhaust would be the first to go, and there would be unmistakable evidence. Has it been inspected? The hose section immediately after the water injection ring? Seizing of the engine would be the LAST thing to go, not the first.
Also, the engine shut down. I don't know of any overheat alarms that shut an engine down. They make noise, but don't shut down the engine. And again, it would take a while for the alarms to go off, and even longer to seize and engine.
Bottom line is that I suspect there is nothing wrong with your engine, and that it stalled because the prop came into contact with the shoal and stalled the engine. I'll wager that your torsional coupling came apart and is wedged in the bell housing, giving the appearance of a seized motor. The transmission will probably have to be pulled back to get a good look, and certainly to fix it. But there might be access holes or ports through which you can see how things look. A diver should also check the running gear.
With no signs of damage to the cooling system, I think it's very unlikely that there is any engine damage.
Just as a baseline side story, when I was a kid, pre drivers license, I had a junker Dodge Dart that I drove around in our fields for kicks. After some repair, I left it full of water rather than a mix of antifreeze, probably because I didn't have the money to buy antifreeze. It got really cold, the water froze, and cracked the block. That was the end of the car, so decided to see how long it would run with no coolant, so off we went. And I drove the piss out of it - no trawler speed operation back then. I was going for a spectacular lockup of that engine. Well, it took a LOT longer than I expected. Proably a good 10-15 minutes, well past billowing coolant (what little remained), almost to the point of boredom for a 14 year old kid. Lesson learned - it takes a lot of abuse to seize and engine.