i'll go with dweste on this.up in canoe country the water is very cold even in summer,in spring it's like ice water.if you go in you could do the reflex gulp and go under.the self inflates seem to be for duck hunters in warmer parts of the country who don't want a PFD to interfere with their gun handling.the only example i have read of at bwca.com involved a tip over and the guy with the co2 PFD could not get his to work.luck was with him as they made it to shore holding onto the canoe,the other guy had a real PFD.the co2 guy tried to inflate his on shore later on and it worked...
my "real" PFD is my avatar and you can see the pockets are filled with a ditch kit.