Deep Ones are aquatic humanoid entities known to both Delta Green and its British counterpart PISCES, encountered by Hex and Hanover in a prior mission. Hex's best friend was kidnapped by a Deep One called Marsh who possessed intimate knowledge of Hex's life, leaving Hex with persistent amnesia and dissociative episodes. Delta Green assets were deployed in response but no after-action confirmation of Marsh's fate was ever received.
In the H-Cell Chronicle, Deep Ones are discussed as a background threat rather than an immediate adversary. Harry Lane confirms they are 'up to something or have been disturbed' near the Dorset coast, and Karotechia documents uncovered by the cell describe a 1942 programme — Operation Blackwater — achieving 'final communion with Dagon' and conducting reproductive experiments with an unidentified element on coastal populations. Haze speculates the documents describe a Nazi-Deep One breeding programme aimed at producing supernatural soldiers, with both the Deep Ones and the Nazis seeking advantage from the arrangement.