Why is the human body mostly water?
Simply because any organism must maintain balance with the environment.
In biology, we use the term homeostasis as a unique characteristic of a living organism to adopt well with the surroundings. The reason why we are mostly composed of water is that we are maintaining a balance with our environment.
When it's hot, we see ourselves sweating. This is because we feel heat, which, as we know may cause uneasiness to our skin, thus we excrete excess water through our skin.
When it's cold, the water molecules contract and our senses shiver, that's why we shiver in cold to produce heat.