You try to fall in the water from a deck and stay less than 4' below the water. If you can do that, then you can pull the manual release.

I know, from personal experience that if you shuffle forward in "vibrant" conditions with "green water" coming aboard; or, are in the cockpit in heavy rain and get pooped, filling the cockpit -- then you will appreciate not having you vest blow-up when you don't want it to.

I have heard that like an emergency life raft, if you have an inflatable PFD that is inflated automatically, then it is suspect because the explosive force is rough on the seams.