While I agree that there should be a shut-off valve in every jet boat, I think that there's more for us ALL to learn here.
The scenario that I imagine is that this guy was out by himself for a simple mid-winter joy ride or a shakedown/tuning cruise. He suddenly notices that the boat has gotten sluggish, or maybe the water got high enough inside the boat that its lapping at his seat bottom.
At this point, PANIC sets in. He realizes he has a no-$h!t 5-alarm emergency! (put yourself in his place). At this point, there's so much water in the back of the boat that he couldn't tell where it was coming in from, even if his heart wasn't racing at 180 bpm. It could be from a lost freeze plug or a loose or burst hose, which the shut-off valve would take care of. It could also be a failed intake seal, cracked suction piece, or a hand-hole clean-out that came loose. There are probably other things I haven't thought of, not related to engine cooling, that couldn't be fixed by shutting off a valve. At this point, the chances were probably very slim that he'd be able to logically diagnose the cause and fix it. I don't think I could.
My point is this: There are many things that can quickly go wrong out on the water, and the cold water compounded his risk. The first thing everyone should do in any kind of situation like this is put on their life vests. I think the urge is to "Save The Boat", when we should more concerned with self-preservation and the safety of our friends and family. He still might not have survived due to the cold water, but I think it would have increased his odds.
A very unfortunate event. It made me think about things I should/could do in a similar situation. Is there anything else we can learn from this???
Be safe out there....