Ambitious evidence-based policies are urgently needed to redirect mankind’s trajectory towards ocean sustainability. While global environmental assessments (GEAs) synthesizing ocean knowledge are multiplying, it is crucial to ensure that their processes and outputs are conducive to social legitimacy, scientific credibility, and are relevant to decision makers’ needs. Building upon institutional and scientific literature, we consolidate a list of best practices for GEAs to achieve legitimacy, credibility, and salience. We then develop a standardized framework to score the level of implementation of these best practices and to assess the coverage of ocean knowledge in GEAs. Lastly, we apply this framework to review 12 influential reports at the ocean science-policy interface. We found that credibility best practices are well implemented but that all GEAs, and in particular ocean-focused assessments, have significant opportunities to strengthen the implementation of legitimacy and salience best practices. We identify that increasing stakeholder engagement and broadening the diversity of knowledge represented is needed to improve the legitimacy of GEAs. Additionally, we found that GEAs could increase their salience by featuring more actionable knowledge for decision-makers, including futures thinking, multi-scale intervention options, and evaluation of progress towards international targets. Finally, we highlight four recommendations to strengthen the GEA seascape: elevating co-production practices, bridging scales through multi-level approaches, increasing transparency in knowledge choices and gaps, and coordinating assessment processes.