The document people are scrabbling for (I think) is this one.....
Probable Sea Survival Times
Basically you don't die from hypothermia, you die because of cold shock or latterly the inability to protect your airway (ie drowning). Table 5 makes for sobering reading.
I wear a membrane type survival suit every day at work for up to 8 hours. Not terribly comfortable but you get used to it. An immersion suit is a different beast and is designed to provide insulation and water proofness and reduces you dexterity to the point you wouldn't be able to fly.
We wear a lifejacket that has a PLB and day night flare. It also has a spray hood (essential).
All that is essential for prolonged over water flight imho.
Liferafts are only any good if they can be deployed and you have had some practice in using them. Launching one off a boat is very different from deploying one whilst you are in the water trying to stop waves washing into your face. Would you go back into a sinking aircraft to recover the raft and where do you attach the lanyard to? That is why modern helicopters have their liferafts fitted externally and all helicopter liferafts have to be deployable from outside. Oh and the helicopter is designed to float for at least long enough to ensure evacuation.
It is my considered opinion that if you end up in the drink wearing just a lifejacket and summer clothes and there is anything more than 10 - 15kts of wind you are pretty much dead. It is unlikely that even a SAR helo will get to you in time.
But the chances of it happening............?