Why is water the moderator of temperature?

For a number of reasons. It is the most abundant natural liquid found; it used in the preparation of food so knowing the boiling and freezing temps are beneficial; it is the basis of weather as its temperature affects things like wind and, of course, precipitation; it has a simple chemical formula as one gram of water equals one cubic centimeter in volume as well as raising that one gram of water one degree Celsius equals one calorie (calories in food and exercising are actually measured in kilo calories or one thousand calories); lastly it is easy to measure.