A growing body of research suggests that genome size in animals can be affected by ecological factors. Half a century ago, Ebeling et al. (1971; EEA71) proposed that genome size increases with depth in some teleost fish groups and discussed a number of biological mechanisms that may explain this pattern (e.g., passive accumulation, adaptive acclimation). Using phylogenetic comparative approaches, we revisit this hypothesis based on genome size and ecological data from up to 708 marine fish species in combination with a set of large-scale phylogenies, including a newly inferred tree. We also conduct modelling approaches of trait evolution and implement a variety of regression analyses to assess the relationship between genome size and depth. Our reanalysis of the EEA71 dataset shows a weak association between these variables, but the overall pattern in their data is driven by a single clade. While analyses based on the new dataset resulted in positive correlations, providing some evidence that genome size evolves adaptively as a function of depth, only a fraction of individual subclade analyses yielded statistically significant results. By contrast, negative correlations are rare and largely non-significant. All in all, we find modest evidence for an increase in genome size along the depth axis in marine fishes. We discuss some mechanistic explanations for the observed trends.