Sam, I agree with Jon. It's the surface temp that makes the most difference for achieving depth. I think a thermocline is only really a barrier for newbies when it is severe, like going from a warm surface temp to 4-6 C. But if the surface is 10C or less, then you either cool down way to fast or have to wear too much neoprene.
Also, I would say that for temperate waters (surface temp), the different zones of lessening comfort would be 17-21+, 12-15, 9-11 and anything below 8 C is exponentially colder with each degree C less.
Oh, I forgot to ask, are you talking about diving with or without a suit?
