42F sounds like my first checkout dive. In an icy lake. When we swapped out regulators, I literally had to use my left hand to find my lips, while I put the regulator in with my right. Not fun.
I think the industry today does itself a major disfavor when they say "Everyone can dive!" because there are some people who are panic-prone. And some situations that can push buttons on everyone. Which is why some agencies used to have a "harassment day" in training, and if someone could make you panic and lose it while in the pool...no certification. Yes, there are arguments that it dangerous too, but a friend spent a day in a chamber after panicking when her primary regulator (diaphragm) burst and flooded, making an uncontrolled ascent. Despite an octopus and a buddy. She was lucky, no permanent damage, but she gave up diving after that.
Thermoclines can be spooky.