I'm not entirely sure that it's rational (actually, I'm pretty sure that it's not entirely rational), but nothing offends me more deeply than the idea of deliberately depriving someone of access to water.
Water binds us all together; it permeates us; it balances us; it is the solvent in which the world's biosystems and ecosystems and weather and geography have formed and thrived. It is the most elemental of substances, in chemical terms the simplest non-gas with which we have any regular contact.
Water is not property, at least not for long. It flows away, it evaporates, it passes through us. Too much is as bad as too little, but both are usually temporary. When it goes away, it comes back; when it overwhelms us, it drains away.
Monopolization of water sources is the most illegitimate form or use of power I can imagine. Shared management of water is the hallmark of our earliest civilizations, and nothing bears starker witness to our loss of humanity than drought as an excuse for power-grabbing and violence.
Related: Heartbreaking evidence of global warming, too.