Im not really sure how cold I can stand or for how long, generally speaking, I dont care for much cold at all, but that alone wont stop me from doing dives. Our OW dives got down to around 60 I think, and that was pretty chilly, but we were wearing shortys (i dont remember the thickness of those, and they were well used). My legs started to cramp up at the end of the 3rd dive, but Im not sure if that was due to the cold or not. Either way it was tolerable for the time we were diving, significantly longer, and I probably would have gotten too cold.
I can imagine cold water down your back is a little uncomfortable.