And just to clarify a few things... Life jackets are not required to be worn unless you are a child under a certain age. Common sense dictates that if you can't swim, you should wear one when out on a small vessel.
The man did put on his life vest. Maybe hypothermia set in and he slipped out of it. We don't know why he died.
I agree. People that can't swim should wear them all the time. You should also put one on if you're going out alone or when the water gets cold (like now). Even a good swimmer has a tough time with heavy clothes weighing them down and body parts going numb.
I was thinking he might not have had a chance to fasten it all the way before jumping overboard. Who knows.