Sorry, I tried posting yesterday or Saturday, but it froze & I had to shut down the browser page & lost all I wrote.
Thermocline runs about half depth to bottom. In 20' that would be from 10' down to bottom. 40' would be 20' down to bottom.
However, it may not apply to shallower water, such as 11 to 13' deep, as I've fished in the No Wake Zone coming out of Running Branch Marina & never encountered any problems in the summer. But I have noticed it in 15' to 16' deep waters.
If you are in an area of heavy traffic- skiers & jet skiers, they can stir up oxygen into the water. However, on the other hand, outboard motors have underwater exhausts, churning carbon monoxide into the water as well. Add a great volume of traffic during the summer.
One article years ago (7 to 15) shared of a guy rod-n-reeling for Big Blues in the summer in lakes during thermocline. He would use goldfish. He said since the thermocline was at half depth, he would set his hook just above or barely into it. He noted the Big Blues suspend themselves just above the thermocline level. And if the hook & bait was slightly lower, a big blue would make a dash to retrieve the bait then get back out of the danger zone