My suggestion is that when you get one, if you can, dive in with it on. I did that, by accident looooong story, but I felt very comfortable in it. Having that "Done there, been that" made me feel very safe. I know, if I go in, my head will remain above water and I'll be comfortable. Of course if you get an auto inflate, you would "waste" a charge. Mine is a Stearns regular old vest type, I looked at others, but it is comfortable for me.
Mike