Water's large heat capacity allows it to reduce temperature swings. Water, for example, has a specific heat capacity that is approximately five times that of sand. When the sun goes down, the land cools quicker than the sea, and the slow-cooling water can leak heat to neighboring land during the night. This is why lakes stay cold even when the weather is warm elsewhere—the heat slowly leaks into the lake until it reaches ice-cold temperatures.
Water also absorbs heat from the atmosphere and releases it during its transition from liquid to vapor phase. This is called latent heat and it makes water a very effective thermal storage medium. If you pump out all the water in a swimming pool and shut off the valve, the temperature will quickly drop below freezing (because it depends on the rate at which heat is lost to the surrounding air). After several hours, if there's no further loss of heat, the water would be frozen. But because there is still some water left, it will not freeze completely.
Finally, water's large heat capacity means that it can absorb large amounts of heat in small amounts of time. This is important in situations where heat is being added rapidly, such as a fire. The larger volume of water that can be brought to a high temperature more quickly means that less water is needed to put out the fire.
Because water has a higher specific heat than dry soil, it absorbs and releases heat more slowly than land. Water's high heat capacity limits its temperature within a relatively limited range, resulting in a restricted daily and seasonal temperature range in neighboring coastal areas.
Water also flows toward cooler regions, so it can have an impact on local climate. Flows from rivers are important for irrigating crops and for washing away pollution, but they can also carry sediment that can dam up streams' channels. The amount of flow in streams depends on their size: Small streams often dry out completely between storms while larger ones tend to flood regularly. Both factors influence whether streams are prone to erosion or not. Climate change may well mean that these river flows become less frequent or even stop completely in some places.
Sea levels are rising because water warms up when you put heat into it. As water heats up, it expands and becomes more dense, which is why liquids such as water are able to float on top of gases such as air or oil. As sea levels rise, this floating layer gets thicker, which causes more heat to be absorbed by the ocean floor. This in turn causes more ice shelves along Antarctica's coast to melt, leading to more water being released into the Southern Ocean and raising sea levels yet further.
The thermal energy that reaches the deep ocean is lost to warming rather quickly.
Because water is a sluggish conductor of heat, it requires more energy to raise its temperature than sand or dry land. In order for the temperature to drop, water must lose more energy than sand (dry land). The ground is substantially warmer than the ocean throughout the summer. This is because the soil retains much of the heat that radiates from the earth's surface.
Water that is isolated from the atmosphere will slowly reach its boiling point, but if the water is part of a large mass like a lake or sea, then it won't reach its full temperature until some of it evaporates. At this point, it becomes gas and rises into the air, leaving behind only cold, dry land. Ocean waters are also cooled by their proximity to land; mountains shadow lower areas and can change the climate there, while oceans tend to take away the heat that is released when continents collide or move close together.
As water heats up, it becomes less dense and floats upward. This is why hot springs are usually found near the surface of the earth, and why deep wells often contain water that is even hotter than that found in shallow ones. As water gets colder, it becomes more dense and sinks downward.
Land has very few natural reservoirs of heat, so it must get the help of other matter to warm itself during cold winters.