Water has two unique characteristics: it is a very poor conductor of heat and it has a great capacity to hold energy. In other words, it takes a lot of energy to raise the temperature of water, but once the temperature is raised, the heat energy is dissipated very slowly.
When sunlight strikes the surface of water, it heats the top layers most readily. The sunlight is transmitted by the water only enough to heat lower levels a little bit. In the ocean, most of the light energy is absorbed in the top 50 centimeters. At 2 meters depth, 98% of the energy has been absorbed and transformed into heat.
Since water has a high capacity to hold on to heat - once heated it stays hot for a time, seasonal changes in ocean temperatures lag about 2 months behind land temperatures. So in temperate climates, the ocean is much warmer for swimming in the fall than in the spring. This temperature lag is even greater in deeper layers of larger lakes and in oceans. The lag can stretch to as long as 5 months (Russell-Hunter, 1970).
Heat is transferred to lower levels of lakes, ponds, and oceans largely by the circulation of the body of water. Water circulation is caused by the movement of water molecules between areas of different densities. Since water near the surface is less dense than water near the bottom, the water circulates, bringing the solar-heated water of the surface down into the body of water and heating the lower levels.
The water near the surface and warmed by the Sun is less dense than water near the bottom, because density of water changes as the temperature of the water changes. The lower the temperature of water, the higher the density of that water—until around 4ºC. At 4°C, water reaches its maximum density. Water below 4°C is lighter than water at 4°C because the water molecules are beginning to form into ice crystals. During the phase change from liquid into ice, water molecules begin to take on a crystalline structure which is less dense than liquid water. This crystalline structure (ice) floats to the surface.