Green’s Dictionary of Slang

(US prison/Und.) a phrase of warning: an officer is making his rounds; a police officer is approaching.

1993		E. Richards Cocaine True 115: There were dozens of dealers out there [...] When a lookout screamed ‘Agua’ or ‘Boombah,’ slang for police, they would escape through the empty houses. [Ibid.] 153: They get themselves thirty dollars a day to tell the pushers when the Agua, the cops, are comin’.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Agua: Spanish word used to warn other prisoners that an officer is making the rounds.