As the importance of the underwater domain in creases for the military user, with Uncrewed Underwater Vehicles (UUV) becoming more and more prominent, so too grows the need for these assets to communicate in a resilient and secure manner. The constrained environment in which acoustic communications operates in, with propagation severely limiting the bandwidth and data rate, means a further reduction in performance usually required by traditional cryptographic standards, such as the Advanced Encryption Standard (AES), is impractical. This paper presents the Phorcys Cryptographic Interoperability Specification, a novel security suite designed to exploit the properties of the underwater channel rather than being contained by it, to provide security without compromising communications performance. Developed jointly by the Defence Science and Technology Laboratory (Dstl) and the National Cyber Security Centre (NCSC) in the UK, with support from NATO Science and Technology organisation Information Systems Panel 174, PCIS is a collaborative effort to address the lack of secure underwater communications standards.