I wear a gas life jacket whenever out on the boat and you dont get in myboat unless you are wearing a bouyancy aid/ jacket of the right type, in good condition and worn correctly.
a. zipped up buoyancy aid with knot or clip used if available
b. with the webbing correctly fitting so that you can get 2-3 fingers through the waist. as tight as you can be comfortably.
If its loose it could go over your head when you wave your arms for help! (do a pool sesion sea servival course to fing out what works)
I use a manual as I dont want it to go off acidentally when driving, and therefore accept the higher risks.
If your buying kit for the less experianced and passengers I would recomend an automatic. They have improved since I bought mine.
If you can get an auto with a spray hood its worth it, as it stops the water jumping down your throat as you lie in a way that the water hits you in the face (you can read up on that somewhere i'm sure)
As for the cold, well you need to get through the first 10 -30 seconds of cold shock,
I don't know the whole science. its not hyperthermia, (that takes a bit of time, its exposure), with cold shock you hyperventerlate as your body automatically tries to warm you by shivering and cutting non core functions and the body needs oxygen so you then can drink lots of water as you dont have much control. Its apparently what kill most people at sea. Certainly in the war and back to the Titanic they knew that.
As a side line, a friend on expedition near glaciers and ice bergs told his group that if they spent 4 mins in the water they would prob be dead, but thats glacier run off at just above 0c and not many people get to do that.