This paper describes the way in which Systems Engineering has been used to map out and address the technical, operational and regulatory considerations necessary for autonomous platform management of Unmanned Surface Vehicles. Building on an approach originally developed for Unmanned Aerial Vehicles, Model-Based Systems Engineering has been used to derive the context and requirements for this high-level ship control system to ensure that it is properly structured, adaptable and re-useable. Mapping out use cases of the platform systems of a large, complex unmanned ship has allowed the functional requirements to be derived rigorously and therefore informs the selection of the most efficient architecture and interfaces ahead of software creation. This practical application of Systems Engineering has paved the way to the creation of robust, open-architecture control of platform systems which enables vessel autonomy in the Naval domain.