When I lived in the North Cascades National Park in the US I frequently had jack all to do except 1. Hike to the top of something tall. 2. Freedive in alpine lakes. And thermoclines were serious stuff and also something I had to contend with. Glacier melt fed lakes are balls cold a couple meters under the surface, even in summer!!
It initially felt like mother nature had set a line and told me not to cross it. A really cold thermocline could bring a sense of real dread. Like the first time you swim out from a reef and over a near vertical wall drop, or the first time you enter a powerful rip current. No amount of knowing it was there ahead of time really made me comfortable with it, I had to become physically acquainted and comfortable with practice. (Like now I get all psyched up because wall drops are where the big ocean animals are so I seek them out, or entering from shore at a rip current I'm all familiar with gets me a free ride out to sea -- haha keep kicking from the boat launch suckers.)
What I found was I needed 2 mm of neoprene EVERYWHERE to tolerate the temperature changes. Like, what seemed like thin gloves made a big difference in my comfort. I had decent cold tolerance, but if I was going to ride the hydroelectric dam current intakes across the dead, empty rock cliffs that used to be canyons I NEEDED COVERAGE to stop the horrible jolts of cold as I went between layers, sometime encountering an unexpected very cold layer that the topography and currents pushed up ino the shallows.
I could handle cold, but my body needed some lag time to respond to it -- which a layer of water trapped on all part of my body provided. My final configuration was a 2.5mm full suit with booties and gloves, and a 4mm vest, hood, and full mask with silicone covering most of my face.
But there are personal tolerance limits. I could go into the water pre-ice during early October (because bats trying to fatten up before winter hibernation are AWESOME as they buzz by you collecting insects at the water's surface at night) but once there was ice on the surface I had maybe, a minute max, before I started getting leg cramps. Even after repeated ventures in, I had reached a personal limit of cold. (I will never become a polar bear swimmer, I guess.) During late fall/early spring, those ice water temperatures could be just below the surface, and I simply didn't cross the temperature boundary, even if it meant a day of surface play instead of diving.
So in my case, I found a way to limit the effect of the temperature shock by making it more gradual. But, there was definitely a limit for me. And I like respecting that limit because I think it's one of those ones where I'd die if I crossed it.
