Ten percent of the world's population depends on the ocean for a readily accessible source of protein and employment. Coastal ecosystems and the communities that rely upon them are facing extreme challenges of increases in ocean pollution, loss of habitat, ocean warming, and changes in ocean productivity. With the whole system under mounting pressure, governments need to scale down food security analyses to the coastal community level to avoid overseeing rising levels of food insecurity. This paper provides an alternative view and analysis of food security at both a national and community level taking into account these marginalised communities. The results propose a refined definition of marine food security and new quantitative methods to measuring direct and indirect reliance on fish within developing countries. Application of this concept and methods reveals that aggregated national statistics mask the extreme levels of dependence on fish for food security in coastal communities within Kenya and Madagascar. The Comoros, Mauritius, Mozambique, and Somalia appear to be the most vulnerable to increasing sea surface temperature, population, and fluctuation in total catch and will be severely affected by a changing Western Indian Ocean from a national, community, and individual perspective. Overall, the study highlights that governments need to disaggregate fisheries data and redefine measurements of food security to more accurately reveal the severity of the potential marine food insecurity crisis at hand.