If you are really getting one for "emergency" use and not a this happens every time I go out, then an inflatable life jacket like this
Might be what you want. I thought that they make a semi inflatable life jacket might be a good option but I can not find one as quickly. Something that you help you out initially that you can pull a cord an it would be a full fledged lifejacket.
The issue with them is that once you inflate them you have a big bulky life jacket for the rest of the day unless you carry an extra C02 cartridge.