As a firefighter we were taught that our turnout gear provides quite a bit of flotation. The heavy coat and pants trap air, the high rubber boots hold quite a bit of air (you have to get your toes up) and the helmet can be held on the chest with a bubble of air in it.
Foul weather gear should do pretty well also. Button the collar up and scoop air under the jacket with your hand (an old Boy Scout trick) to increase buoyancy. Even wet the clothes will be pretty close to neutral buoyancy and the added insulation will be very important in cold water.