The main theme of The Big Thirst is that water is so ubiquitous that it becomes invisible, yet remains the most important resource. Water is necessary for agricultural and industrial production, in homes, and throughout the environment.

From ancient to modern times, water has formed the basis of human society. Recently, water has become safe through filtration and chlorination. As a result, people now take water for granted.

When water becomes scarce, as in a drought, people realize how important it is. Water abundance has been the assumption on which many modern societies have been built.

Because of how common water is, people often take it for granted. Fishman argues for people to take more notice of water, so that they waste less and conserve more.

Water is heavy and hard to transport, and also cheap, so water is not traded as other resources are. Also, because humans depend on water, Fishman argues that water differs from other resources.

Water is so common in the environment that even relatively large conservation efforts pale in comparison to the amounts of water in floods, rain, evaporation, and other phenomena.