We examine species–area relationships (SARs) in the sea, a realm that is structured in fundamentally different ways to terrestrial systems. For example, the open seas and the benthic communities on their bottoms represent the largest ecosystems of the world, but are well connected due to the current systems and the presence of few barriers. This enables a considerable dispersal rate for many marine organisms, which subsequently has a high impact on the SAR in these systems. We provide some examples of studies in which marine SARs have been examined over very large spatial (latitudinal) scales and discuss why patterns in the marine realm might not follow terrestrial expectations. We also discuss some of the problems and limitations of constructing SARs in the marine realm and more generally. We argue that molecular tools probably represent the best opportunity for more detailed and uniform approaches to assessing sampled biodiversity in the future, particularly in the microbial realm, but this is not guaranteed. It will require a great deal of standardization in methods and procedures and a more detailed reporting of these procedures than is commonly the case today.