Having had the dubious pleasure of being "dunkered" every couple of years, plus wet winched at the same sort of periodicity, I can vouch for the fact that there is no substitute for doing it in the cold sea. I did dinghy drills every 6 months (IIRC) in a swimming pool, and it was far from realistic, in fact it often turned into a bit of a laugh.
One SurvO decided to make sea drills a bit more realistic, by dropping us about 5 or 6 miles offshore and leaving us alone for a while. All went well until the SAR diver did his usual dirty trick of turning the one man life rafts over unexpectedly, dumping us all back into the sea one by one. It was April, and the sea temp was around 9 deg C.
One of my colleagues had the misfortune to have the CO2 bottle pulled off his raft, causing it to deflate. He struggled for about 5 minutes or so to board the soggy mass (they stay afloat from the floor and canopy inflation) before giving up. Two of us paddled over to help him out, only to find that he appeared to be drunk, was giggling and couldn't care less whether he got into the raft or not. We sorted him out as best we could and pinged off a miniflare to call in the helo, as the guy was clearly hypothermic.
Bear in mind that we were all dressed for the occasion, with thick underwear, wooly bear, immersion suit flying gloves and helmet and this chap had only been in the water for ten minutes tops.
Those flying over UK waters in shirtsleeves really don't stand much of a chance of being able to help themselves I think. Certainly I always wear a survival suit, even for relatively short water crossings.