Per our beloved belM: thermocline was not just a difference in temperature of the water but the visible line that is created where the different waters are separated.

I find that the line is a lot easier to see in a lake where there is much less wave action than in the ocean. Most often the waters even have different clarities.

This line isn't always visible, but it is the line that divides a mass of water of one temperature from a mass of water of higher or lower temperature. My experience with it is from diving - typically when you're in the water the thermocline isn't visible, but sometimes you can really tell when you cross it - get 30 or 40 feet down and bbrrrrr!!!