Environmental Impact Assessment (EIA) is a well-established practice in most developed countries, even though its application to projects in the marine environment is at a much earlier stage of development. We use the Portuguese example to address marine EIA legislation since its exclusive economic zone (EEZ) is currently the third largest in the European Union and its EIA legislation does not require various offshore activities with potentially negative environmental impacts to undergo EIA before being licensed. This paper aims to determine whether three types of projects implemented within Portuguese maritime zones – artificial reefs using sunken ships, hydrocarbon prospecting and wave-energy generation – would benefit from application of an appropriately designed EIA. We have conducted a structured review of EIA legal provisions from seven other countries, and considered whether a full EIA was required for each project type. Consequently, 12 Environmental Impact Statements (EIS) have been compared to identify patterns of (dis)similarity across countries and project types. Additionally, we identified key descriptors and predicted impacts for each project type referred to in their EIS. The main conclusion is that ultimately all three projects would benefit from mandatory EIA in Portugal. This paper is relevant for countries with large maritime areas and underdeveloped marine EIA legislation, helping improve international policy-making relating to these three types of marine projects.