Yup, attacking water systems is a violation of the Geneva conventions and a clear unlawful order of which members of the military are compelled to not follow as they are bound by the Geneva Conventions and trained on it.

Protocol I (Article 54) and Protocol II (Article 15) of the Geneva Conventions prohibit attacking, destroying, or rendering useless objects indispensable to civilian survival, such as drinking water installations, dams, and dykes.

Article 33 of the Fourth Geneva Convention prohibits collective penalties and all measures of intimidation or terrorism. Threatening to cut off electricity and water to an entire population as a means of coercing political compliance clearly constitutes collective punishment.

Destroying Iran's electric and water supply is a war crime.