In circumstances such as a ditching there is a saying, if it's not strapped to your body you're not going to have it if you successfully exit.
There are circumstances in which a lifejacket is merely going to prolong your death. Availability of rescue services, did you get a MAYDAY out - if you didn't no one is going to come looking, until perhaps Mum realises you're late for tea. In rough seas you need a jacket with a spray hood, else you'll drown from wind driven spray. Temperature of the water - if it's cold, cold, best have a dry suit.
Cold water saps the life out of you darn quick.
You would think that water temp in the 70–80° F (21–27° C) range would be fairly benign. Yet one reference gives 3–12 hours to exhaustion or unconsciousness, and 3 hours – indefinitely expected survival time. You the 3 hour or indefinite?
More to consider than just having a jacket.
https://www.ntsb.gov/news/events/Doc...e2-Hiscock.pdf
http://www.dtic.mil/dtic/tr/fulltext/u2/a305730.pdf