Marine biodiversity is crucial to the health of our oceans and ecosystems, but accessing and utilising the data necessary to monitor and protect marine life is an ongoing challenge. Across Europe, barriers exist that prevent seamless data flow, hindering the effective use of critical biodiversity information. These barriers, ranging from technical obstacles to governance issues, prevent the full potential of marine data from being realised. The DTO-BioFlow project aims to break down these barriers by providing a comprehensive guide to understanding and overcoming the challenges of marine biodiversity data access, integration, and use. This document represents the first version of the Playbook which identifies the main barriers and focuses in on the specific obstacles that are most challenging in enabling data to flow into the pathways developed within the DTO-BioFlow project, and highlights which types of data require the most effort when trying to on board to the European Digital Twin of the Ocean (EU DTO). The final version of the Barriers Playbook is scheduled for release in Autumn 2026.