Well, like Andre asked, I can tell you about my 12 hours in zero-temperature water. The case was actually started by a stupid article in magazine, in which some reader insisted that my earlier article about survival suits (Gore-Tex RIB models) was false. He said, that the suit can't keep a person alive more than 2-3 hours even if worn properly. So I made a test on April here in Finland, it was something like -2 C air and icy water. I spent 12 hours in water, it was so could that water froze over my Crewsaver lifejacket and the suit was also covered with ice. I survived the test easily, the most difficult thing was to keep fingers warm enough during the day. I had only ordinary neoprene gloves with me, no dry-gloves. It took me 3 weeks to get back "touch sense" to my fingers, but after all no permanent damages occured. I'd say that Gore-Tex suit keep you alive at sea 24 hours even in the coldest conditions you can find. My 12 hours was easy, but if your only target is to stay alive, you can easily take 20 or maybe 30 hours depending on personal cold resistance. The major problem is urination, because body developed some 800 ml of urine every day and you should get rid of that by some way. Otherwise the suit will get wet inside because you just can't keep all the urine in your bladder till someone finds you drifting at sea.
I can send couple images when I come back, but the test was pretty simple. The story was covered in Finnish boating magazine about two years ago with 5-6 pages.