I almost always wear a lifejacket when flying in the Scottish Highlands. I doubt my ability to get one on securely in a sinking aircraft, or in the cockpit gliding down.
Over land, a loch (near the shore) would often be the best option for puting down if the engine failed.
From dinghy sailing and canoeing in winter many years ago, without wet suits or survival suits, the effect of cold water immersion varies greatly from person to person. Wet wool gives some insulation. I'd keep my footware on, for insulation.
Fortunately my usual aircraft has a poorly heated, draughty, cabin, so I dress warmly.
There's a U.S. site somewhere listing statistics for survival of ditchings near land. Survival is very likely, if a lifejacket is worn.