I did a dive in 42F water last year. I was in my dry suit and called the dive after 25 minutes because my lips and my fingertips got too cold to be in the water any longer. Followed it with a 45 minute dive in 52F (not as deep so didn't hit that last thermocline). The temperature was much more tolerable. Personally, I wear a dry suit for 95% of my dives. The water was to be in the high 70s for me to consider a wet suit. But I'm a wimp when it comes to cold water.