The 1972 World Heritage Convention (WHC) and 1994 Global Strategy aim to preserve the outstanding universal value of internationally important cultural and natural sites within a “representative, balanced and credible” network of highly-protected areas. Increasing human pressures and shortfalls in representation have been documented across the World Heritage network, particularly in terrestrial and cultural sites, threatening the integrity and primary goals of the WHC. However, the conservation status of current and tentative (i.e., proposed) marine natural World Heritage areas (mnWHA) remains relatively unknown. We assessed the extent of recent (2013) and historical (2008–2013) cumulative human impacts and several metrics of representation (country, continent, ecoregion, wilderness, and ‘at-risk’ species) within existing and tentative mnWHAs. We found moderate yet increasing cumulative human impacts across most existing sites, and high or very high impacts across the majority of tentative sites. Climate change impacts comprised nearly 75% of impact scores, on average, and differences between land- and marine-based impacts across sites could help prioritise management decisions. Over 75% of marine ecoregions and 80% of ‘at-risk’ species considered in this study have no representation within the existing sites. We outline how prioritizing representation across tentative sites for future World Heritage listing could greatly increase these metrics. We urge the WHC to adopt quantitative, systematic and transparent evaluations of how current and tentative sites contribute to the overarching goals of maintaining a representative World Heritage network and preserving outstanding universal value for future generations.