Approximately 0.5–2.0 million eukaryotic species inhabit the seas, whereas 2.0–10.0 million eukaryotic species inhabit freshwater or the land.  Much has been made of this several-fold difference in species richness but there is little consensus about the cause(s); nonetheless, the idea of an elementary distinction between marine and terrestrial systems has become pervasive.  Here, I present three perspectives on the generality of ecological and evolutionary processes, principally from the seas.  First, I point out that, by some metrics, differences within realms can exceed those between realms; supposedly fundamental differences between sea and land are overly simplistic binary representations of more complex, theoretically rich, suites of comparisons, for which relevant frameworks are being developed.  Second, I note that comparisons that lack a relevant framework have over-emphasized unpredictability in some processes (e.g. dispersal) within realms, which itself has hindered discovery of general processes across realms.  In contrast, for example, when comparing genetic structure in synchronously diverging co-distributed species, using standardized analyses of eastern North Pacific rocky intertidal species, differences in the number of propagules and their pelagic duration (PD) are sufficient to explain measured differences in population genetic structure (FST).  A quantitatively similar relationship between PD and FST for co-distributed species from the Great Barrier Reef suggests general, perhaps predictable, interactions between species’ traits and geographical variation and their effect on population structure.  Third, I describe patterns of species diversity in marine lakes in the context of island biogeographic theory.  I find that species richness is related to area and colonization distance, and that community similarity is related to environment, i.e. marine lakes show patterns that parallel those for terrestrial islands.  These results may indicate greater generality in ecological and evolutionary processes across aquatic and terrestrial environments than usually previously inferred.