Sounds good, doesn't walk far. There are many shoulds never put into practice.
Anyway, I don't spend much time on the water actually. I was probably over confident in my swimming skills when I did more, but I survived those. Some confident swimmers all too commonly don't survive boat mishaps. Over 500 last year:
http://www.uscgboating.org/assets/1/Publications/2011BoatingStatisticsreport.pdf
Put a diver in mask, fins and snorkel, as if the boat sank ( say in the space of 5 minutes it went down).
What possible excuse is there for a DIVER not to be able to either swim to shore( if it is within 5 miles and there is no outgoing tide or current) OR, to stay horizontal, relaxed, and breathing comfortably through the snorkel for even an entire day, if not longer.
For a huge percentage of divers out on boats, there is the potential for swimming to shore, or to an area where many other boasts could be found( potential rescuers). The pfd wearer floats on the surface 100% helpless and generating no body heat from swimming--meaning hypothermia is much more likely to visit the pfd wearer, much sooner.
As relevant to this thread---the safety of divers on a boat entering an area of some danger, and how to best ensure their continued safety....Should the Captain tell divers to don pfd's in this case, or would this be foolish, given MUCH BETTER OPTIONS open to "divers". And again, if anyone thinks the danger to protect against is unconsciousness, that is NOT what a pfd is made for.....for this you need a bike helmet, a whitewater kayak helmet, or a football helmet type solution. This will likely PREVENT your becoming unconscious. An unconscious person in the water with a pfd could easily drown, particularly if they were in a boat that just flipped over. So...Either protect your head so you can take a huge hit and stay conscious, or, find a way to avoid the hit.