Not only should it be an inflatable, but unless you are riding an ejection seat, do not get a water activated inflation system.
Another way to go is with the
Switlik LPU-32 . Take the vest of your choice and simply wear the LPU-32 in addition. Available on eBay -
NEW-SWITLIK-LPU-32-P-US-Military-Survival-Life-Preserver