I can imagine it would be chaotic if the seas were that big. Leaving a boat turned off would leave you at the mercy of the winds and waves. Maybe not do the dive? If you are not use to those sea conditions, isn't it up the the diver to call the dive and not the boat? As a diver, wouldn't a running engine be a clear and obvious danger? Hard to place blame on diver or boat with no solid info.
I have done lots of stupid dives in my life....diving since 1972 , and exploring all the deep 200 to 300 foot stuff off of Fort Pierce to North Miami from 1993 to 2000....
In the 80's it was mostly 100 to 145 foot stuff in big currents off Juno/Jupiter , and no weather was too rough to spearfish, and I did a great deal of diving in 10 foot seas and bigger, off of Palm Beach with Frank Hammet..winds were often big as well. Frank was pretty much the craziest dive operator Palm Beach ever had.
So yes..if conditions are crazy, I can imagine the boat a bit concerned about totally losing power...though most could imagine doing it for the 5 seconds it takes for a buddy team to go off the platform....
For the sake of argument, say this boat was terrified of getting out of gear......if that was all it was, someone at DM level, that has done at least some drops in rough seas, would want to swim away from the boat rapidly...and certainly, not just lay there right by the back of the boat--I see only an OW student doing that, and then only a bad one....
If the boat was motoring forward, and you jump off, you are rapidly far behind the boat...You jump back and away from it as well...the inertia, and the prop, keeps the boat moving away from you. There really is no other way this can work....
But...if the boat was dropping you on a "place" or structure like a wreck or specific reef...and marked the structure.....you could be waiting on the platform....the boat might then reverse engines because the boat had continued forward over the mark, and the captain wanted to get the divers back over the mark....and then..if the engine throttled down, but the prop was still engaged in reverse...all bets are off....even at low throttle, the boat is coming after the DM, every second getting closer. Unless he KNEW the boat was in reverse and after him, it would be too late before he was impacted, and realized what was happening....Just my take, but it makes sense this way.