Underwater cybersecurity is the specialized domain of cybersecurity focused on the protection of systems, data, and operations within the underwater environment. This field investigates vulnerabilities, threats, and risks targeting underwater networked systems (e.g., Internet of Underwater Things, autonomous vehicles, submarine cables) and associated infrastructure, considering the unique challenges of limited connectivity, the dynamic physical environment, and critical national security implications. Its significance lies in developing strategies for anomaly detection, data privacy, and defense mechanisms to ensure the resilience and integrity of underwater operations and critical infrastructure against adversarial actions.