Water is able to maintain the temperature of living organisms because it has highest specific heat. Specific heat is a physical property of matter. If specific heat is high then the matter does not get heated easily but once it is heated it does not cool down easily either. In short we can say that water can easily resist temperature changes because it has very high specific heat. This property plays a great role in maintaining the temperature of a living organism's body because all organisms have water in their body. When heat is generated within the body, it is unable to disrupt internal temperature of the organism's body because the water does not get heated easily as a result of which the temperature of the body does not rise. On the other hand, the internal temperature of the body remains the same with slight decrease in the temperature outside because water does not radiate heat easily either. This is how high specific heat of water helps in maintaining a constant temperature in the body of organisms.
