In complex underwater operations, the offshore subsea industry can benefit greatly from a novel class of extralarge uncrewed underwater vehicles due to their increasing autonomy and adaptability. However, their reliance on IT, reliable communication, and sensor information poses significant cyber security challenges exacerbated by rising maritime incidents. This study systematically analyzes the cyber attack surface of those underwater vehicles, assessing and ranking potential threats, and identifying critical system components. The research contributes by design to the definition of tailored security requirements. Our threat analysis creates the basis for simulation and test environments that foster the development of appropriate security measures in order to defend against the growing threats from cyberspace and the electromagnetic spectrum.