Doug Kanter wrote:
Assuming:

1) The seas aren't wild, dangerously cold, etc
2) Victim is wearing a decent PDF, is not drunk, and does not lose
consciousness as they leave the boat
3) PDF works properly and victim is floating face up

....how does one drown when wearing a PDF? I'm sure there's an easy answer
here, but I'll ask anyway.

Under the ideal sea conditions you cite, it would be difficult to drown
while wearing a pfd. People have drifted for days in calm, warm waters
and been eventually resucued. More likely will die of thirst or
exposure than drowning.

Up this way, those who go overboard without getting knocked
unconscious, etc, don't have long to live even if they are floating
face up. Hypothermia kills some folks in as little as 15-20 minutes
(depending on body type and the amount of clothing worn) and few will
survive for much more than an hour or so. (Think of the scene in
"Titanic" where all those folks wearing lifejackets slowly turned blue
and expired in the icy water). The cold works so fast and so
thouroughly that MOB's often lack the physical abilty to assist in
their own rescue after just several minutes in the water. Falls under
your category, "dangerously cold".

You don't even have to be immersed. I know of some guys who were
becoming hypothermic sitting atop an overturned trimaran hull while
waiting for rescue
of the W coast of Vancouver Island. They had been wet, obviously, after
the boat flipped but they were all able to get and remain out of the
water. After a night of it, (even a summer night), they were beginning
to become hypothermic. Fortunately, that story has a happy ending. :-)