If the prop was turning the boat was moving even if it was sinking. Ever try to out swim even a slow moving sail boat? You'll be surprised how fast they move when you're in the water. My guess is the boat ran over him.
If he had any training in water craft safety he would have shut down the engine before abandoning ship, in fact, almost ALWAYS the best course of action is to stay with the vessel. Small vessels are designed not to sink.”
100% correct you always try to keep the boat afloat even stuffing your clothing in the leaks that beats being in the water every time and why the engines were not shut down is Curious ,I have been on sinking boats and the worlds best bilge pump is a scared man with a bucket.
Even true for vessels that don't have built in flotation.
There's been many cases where a ship was abandoned at sea during a storm, only to be found still afloat after the storm had passed, surviving the storm on its own.
As for the fellow who jumped, unless there's something the story omitted (e.g. boat might have been capsizing while under weigh, as when turning too sharply for the speed, and he tried to jump clear), he probably just panicked.