War Water is Iron Rust in water along with other ingredients that are added according to the desires of the Spellcaster. Mars... the Roman God was the God of War and his metal was Iron.
Additional ingredients included in War Water to make it stronger are magnetic sand and a rusty nail or coffin nail. Spanish moss provides the proper darkness to the rust color of the mix and gives an appropriate "swamp water" scent when you break the bottle open. To strengthen War Water add nails from specific locations such as a hospital, to cause injury; a court house, to make someone lose a court case, a jail, to get someone arrested, the person's job site, to make them get fired.