Drowning without a life-preserver would be the least of your problems!
In those kind of water temperatures survival is measured in minutes, with or without a life-jacket. Thermal shock, resulting in water inhalation and possible cardiac arrest, is the biggest killer when the body is immersed in very cold water.
Yes, you drown because your head is not held above the water and the gasp reaction sends you under in 30 seconds or if you survive the first minute you drown due to inability to move your limbs to keep your self afloat.
So the life vest is the most of your worries as it keeps your head above water when your limbs give up! Cardiac arrest on entry is rare and lifejacket gives you many minutes more survival time as you are not dissipating your body heat by moving your extremities. Survival at sea 101!
The aircraft was an EOW A 320 and had life vests. The people who are not wearing theirs, are the people who did not watch the flight attendant demo.
You are probably wrong. I've yet to hear any passenger account that mentions they were warned by the pilot that they were going to ditch ("brace for impact" has been reported) or any account that cabin crew told them to don life vests.
It hasn't been confirmed that life vests were available on this aircraft.
Mickjoebill