Within the universe of what is commonly referred to as systemic climate change litigation,1 national courts are typically asked to determine whether the State has met its duty of care toward its citizens as it pertains to the negative impacts of climate change. Significant negative impacts from climate change are frequently driven by ocean-related changes such as sea level rise, ocean warming and the ocean’s role as an integral part of the climate system. Even so, the focus in systemic climate litigation is on climate law rather than ocean-related legal arguments. National courts, when faced with systemic climate litigation, tend to make duty of care determinations based on the State’s conduct through the lens of due diligence. Due diligence is also the lens through which States’ obligations within both the international climate change regime and the law of the sea regime, which governs the ocean, are interpreted. The objective of this paper is therefore to explore how due diligence is articulated within each of these two international legal regimes. Section I provides a contextual overview of the importance of due diligence within national systemic climate change litigation. In Section II, the strengths and weaknesses of both legal regimes are compared through the lens of due diligence in order establish the potential applicability of inclusion of law of the sea regime in national climate litigation to drive increased climate ambition. Section III concludes with a brief look towards the future.