The USGS Water Science School
Heat Capacity of Water
Water has a high heat index—it absorbs a lot of heat before it begins to get hot. This is why water is valuable to industries and in your car's radiator as a coolant. The high heat index of water also helps regulate the rate at which air changes temperature, which is why the temperature change between seasons is gradual rather than sudden, especially near the oceans.
Lucky for me, you, and our fish in the pond, water does indeed have a very high specific heat capacity. The heat of water is the amount of heat needed to raise its temperature a certain amount. One of water's most significant properties is that it takes a lot of heat to it to make it get hot. Precisely, water has to absorb 4,184 Joules of heat for the temperature of one kilogram of water to increase 1 degree celsius (°C). For comparison sake, it only takes 385 Joules of heat to raise 1 kilogram of copper 1°C.
If you leave a bucket of water outside in the sun in summer it will certainly get warm, but not hot enough to boil an egg. But, if you walk barefoot on the black asphalt of a street here in Atlanta, Georgia in August, you'll burn your feet. Dropping an egg on the metal of my car hood on an August day will produce a fried egg. Metals have a much lower specific heat capacity than water. If you've ever held onto a needle and put the other end in a flame you know how fast the needle gets hot, and how fast the heat is moved through the length of the needle to your finger. Not so with water.
Why heat capacity is important
The high heat capacity of water has a great deal to do with regulating extremes in the environment. For instance, our fish in the pond is indeed happy because the heat capacity of the water in her pond above means the temperature of the water will stay relatively the same from day to night. She doesn't have to worry about either turning on her air conditioner or putting on her woolen flipper gloves.
This same concept can be expanded to a world-wide scale. The oceans and lakes help regulate the temperature ranges that billions of people experience in their towns and cities. Water surrounding or near cities take longer to heat up and longer to cool down than do land masses, so cities near the oceans will tend to have less change and less extreme temperatures than inland cities. This property of water is one reason why states on the coast and in the center of the United States can differ so much in temperature patterns. A Midwest state, such as Nebraska, will have colder winters and hotter summers than Oregon, which has a higher latitude but has the Pacific Ocean nearby.