Why is the specific heat capacity of the human body close to, but somewhat less than, that of water?

Because most of the human body (about 70% of it) consists of water

Explanation:

The main reason why the specific heat capacity of the human body is very close to that of water is that water makes approximately 70% of the substances of which the human body consists. However, the human body has a slightly smaller specific heat capacity than water, because it contains also other elements/substances, and since water is one of the substances with the higher specific heat capacity at all, the specific heat capacity of the body is a bit smaller.