The importance of our seas and oceans to the economy and societal well-being is broadly acknowledged. In addition, offshore infrastructure in the form of ports, wind farms, aquaculture facilities, natural gas pipes, etc. has continuously expanded and has become more commonplace in recent years. Activities associated with seabed mapping, monitoring of the health and status of marine habitats, offshore infrastructure inspection, seabed mining and underwater sensing have traditionally been based on the use of crewed support vessels which are expensive to run and have limited endurance. The MERLIN project seeks to exploit long-endurance operational capabilities offered through the use of hydrogen fuel cells and renewable energy installed onboard Unmanned Surface Vessels (USV) and Autonomous Underwater Vessels (AUVs) which are capable of navigating and operating autonomously based on AI algorithms without the need for human intervention. A Mission Remote Control Centre (MRCC) will permit data from the autonomous vessels to be transmitted to base. Conversely, the MRCC will allow the transmission of commands from the supervisor to the robotic vehicles. The vehicles will incorporate advanced surface and underwater grasping capability for the collection of samples, handling, installation and recovery of sensors using custom-built robotic arms. The USV will provide geotagging reference data to the AUVs when they operate underwater and be able to track them during the mission. The USV will be able to navigate from its base to the location of the mission where the AUVs will be released. At the end of the mission the AUVs will dock again with the USV so they can be safely returned to base. The vehicles will be capable of operating independently as well as in combination with support vessels . The demonstration activities include three different high value use cases, including marine habitat monitoring, underwater volcano seabed mapping, and port infrastructure inspection.