Water makes up around 55% to 78% of the mass of a human body and even higher percentages in many other organisms. It plays crucial roles in:

* Solvent: Dissolving and transporting nutrients and waste products.

* Temperature regulation: Maintaining a stable internal temperature.

* Chemical reactions: Acting as a medium for many biochemical reactions.

* Structure: Providing support and shape to cells and tissues.

While other molecules like proteins and carbohydrates are important, water's sheer volume and diverse functions make it the most abundant and vital molecule for life.