very interesting article on the sometimes-fatal results of fallling overboard in cold water even w a PFD and/or drysuit on:
www.practical-sailor.com
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Cold Water Survival - Practical Sailor
Originally known as sudden disappearance syndrome, cold water shock has been known since the 1970s. Different from true hypothermia, which results from the body core temperature dropping over a period of 20 minutes to hours, cold water shock is immediate.

PS - practical sailor allows you to sign up for its newsletter even if you are not a subscriber. good for them !