This is because the surface of the water absorbs heat from the air and the sun! Heat from the sun is transferred to the pool via radiation, and heat from the air is transferred to the pool via convection. However, the heat transfer can only occur at the surface of the pool, at the top. This is because heat transfer can only occur across boundaries. In this case, the boundary of the pool is the top layer of water. This surface layer is in contact with the air, which allows it to exchange heat energy with the atmosphere. Also, the sunlight shines on the surface of the pool, which means that it is also the boundary for radiation. This means that the surface of the pool heats up first.

The bottom of the pool isn’t in contact with the air, and it doesn’t receive direct sunlight, so it stays cold. If you let the pool sit still for a long time, some of the heat from the top of the pool will go to the bottom. This is due to diffusive heat transfer. Basically, even if the water does not look like it is moving overall, the water molecules are still bumping into each other and moving around in the pool. The water molecules at the surface which are moving faster due to all the heat energy they have absorbed from the air and sun begin to bump into water molecules deeper in the pool. They transfer energy through these collisions, and eventually the heat begins to reach the bottom. However, the bottom of the pool will never be the same temperature as the top of the pool. Why is this? The water at the bottom of the pool is in contact with the cement and ground! So just like the top of the pool exchanges heat with the air, the bottom of the pool exchanges heat with the ground. So the ground will absorb heat from the bottom of the pool, and the bottom of the pool will never be able to get enough heat to be the same temperature as the top of the pool!

This phenomenon, where one end of a substance is hotter than the other, is what scientists call a temperature gradient.