Generally depending on your sensitivity settings there will be a weak, thick broken line with heavy shading under it. If it's a strong one, it will be distinct solid line and darker underneath it. On most lakes you'll notice that there a very few fish below this line. Also with a strong one the surface temp might be 90* and down to the top of the line it might be 85*, but below it the temp might be 65*. And yes some lakes will have a strong line and fish below it, these lakes are generally clearer lakes that have excellent light penetration which allows strong plant growth, thus raising the DO level and allowing the fish to live there. Those fish if caught generally can't go back down for a few different reasons.