If wearing warm clothing, some people have a reasonable survival time in cold water. Experience from winter dinghy racing in a small Scottish loch, with ice at the edges, and several capsizes, without a wetsuit. I was in my 20s then, so probably more vulnerable now.
A survival suit would be a lot of bother for every flight, with the likelyhood of ditching close to shore being greatest.
I once forgot to put on a lifejacket before a flight across the Minch. I contemplated landing on a farm strip, but as it was calm, I loosened my seatbelt and put it on in the cockpit,well before reaching the west coast.
On landing, I found the diagonal trapped in it, so I would have had to undo the lifejacket to get out.