The Blue-Cloud 2026 Second Federation Workshopwas organised in conjuction with the Digital Twins of the Ocean Interoperability Workshop (from ILIAD project). The workshop took place on 16th June 2025, as a co-located event of OCEANS 2025 Brest, France (https://brest25.oceansconference.org/) To achieve the ambitious goals of the European Green Deal and Mission Restore Our Ocean and Waters by 2030, new capabilities and innovative technologies like Digital Twins of the Ocean should be promoted and widely adopted, whether at local and national levels, to solve specific challenges, or at a European and international level. By creating virtual, data-rich representations of marine ecosystems, digital twins of the ocean can support decision-making processes that are vital for achieving the European Green Deal’s goals of sustainable resource use, biodiversity preservation, and climate resilience - and also include the additional use of socio-economic and socio-ecological data and models. This innovative technology also aligns seamlessly with the Mission Restore our Ocean and Waters by enabling precise tracking and mitigation of ecological pressures on marine habitats. The workshop will include both one demonstration oriented solution track and one technical services track and joint sessions for both tracks. Introducing a federated and interoperability focused approach, where research infrastructures and technological providers make their services available to the community via agreed data management principles (GEO, FAIR, CARE, TRUST) and Open Science technology, can add that extra contribution to the sharing of knowledge, ultimately stimulating uptake of services and the development of new tools and products. The services can include one or more steps of a digital twin pipeline from observations and sensors, to data management through data spaces and data lakes including support for semantic interoperability, data discovery and access, to digital twin engine and processing services with orchestration of workflows and support for virtual research and analysis environments with AI/Machine learning and predictions/what-if analysis to visualisation and interaction. Official event page: https://blue-cloud.org/events/blue-cloud-second-federation-and-digital-twins-ocean-interoperability-joint-workshops