AQUARIUS will provide a highly comprehensive suite of integrated research infrastructures appropriate to addressing significant challenges for the long-term sustainability of our unique oceans, seas and freshwater ecosystems. For the first time, diverse research infrastructures will be combined to facilitate the work of researchers and key stakeholders focused on challenges and opportunities for both marine and freshwater systems. An impressive range of 57 research infrastructure services will be made available to include research vessels, mobile marine observation platforms, aircraft, drones, satellite, sensors, fixed freshwater and marine observatories and test sites, experimental facilities, and sophisticated data infrastructures. AQUARIUS will support the development phase of the EU Mission to Restore our Ocean and waters by 2030, the Sustainable Blue Economy Partnership, the European Green Deal, and international climate initiatives. It will also be an essential component in achieving the European Digital Twin of the Ocean and the UN Decade for Ocean Sciences. The needs of researchers will be met through a robust and transparent system of transnational access funding Calls, facilitated by centralised user-friendly access portal. The Call programme will be informed through stakeholder engagement and brokerage events. Projects to be selected for Access must convincingly integrate multiple infrastructures and contribute to the core policy objectives of Mission Ocean, that is, to protect and restore marine and freshwater ecosystems and biodiversity; to prevent and eliminate pollution of our oceans, seas and waters; and to ensure a sustainable, carbon-neutral and circular blue economy. A thematic and geographic focus will be the hallmark of the proposed transnational Calls, aligning with the Lighthouse Regions, that is, the Baltic and the North Sea Basins, Black Sea, Atlantic/Arctic, and Mediterranean Sea along with their associated rivers.