I took a boater safety class with my 12 year old early this spring...the Sheriff giving the class said 4-6 lbs of flotation was all a (conscious) adult needed...he gave examples of using potato chip bags, small coolers, zip-lock bags, etc to use for flotation if your boat capsizes and you don't have a life preserver...
It might be ok until the man/woman overboard is wearing full winter gear; boots, thermals, gloves, cap, jackets etc.... and possibly has fishing gear also attached. At this point you become quite a drag in the water. I have the big ones myself.