Surviving w/o a pfd
While a pfd is the best option by far, all of you wading fishermen out there should not ignore the great life preserver you are wearing on your legs - your waders.
By striping off your waders - sacrifice your boots to the deep - and placing them behind you, you can then pull them quickly over your head and into the water in front of you thereby trapping air in them. You can then inflate them further with a few lungfulls of air. It may look funny but they will provide a serious amount of bouancy - you can even sit on them by stradling the crotch while you wait for help. This also works with long pants, though not as well, if you tie knots in the legs.