During many scuba dives you have properly experienced this, but not sure what exactly it is??
Water forms different layers based on temperature, which can be abrupt in very calm water. You can actually be diving in 29 degrees by the Phi Phi islands here in the Andaman Sea and stick your hand down and feel the difference in temperature to 27 degrees .
Sometimes you can even see this appearing on a distance in clear visibility; it is like a glittering rising from a hot asphalt road, caused by a mix of 2 different temperature layers.
You can find this in both fresh and salt water, lakes, quarries and ponds during summer. It forms due to the summer sun, which heats the surface’s water. Due to the density difference between warm and cold water, the cold water sinks down while the warm water rises. A clear, strong border exists between the two bodies of water with the thermocline in between.
Sometimes it can be very hard to predict whether the visibility will improve or decrease below the thermocline. Actually thermoclines are complex and hard to predict. At different points of the sea several meters apart on the same day, the depth, thickness, and temperature of the thermocline can vary significantly. Not all of the causes and reasons for these variations are known, but they can be caused by winds, currents, and other dynamic forces.
Did you know? You will never find a temperature lower than 3 degrees below a thermocline. Because liquid water is densest at this temperature, and the density begins to decrease again from 0-3 degrees.