The colonization of the water column is among the most important transformations in the evolution of animal life and global ecosystems. The Devonian Nekton Revolution (DNR) has been identified as a major macroevolutionary event signifying the sudden occupation of the water column by independent radiations of swimming animals. Using new data, an expanded taxonomic coverage, sample standardization, and increased ecological resolution, we reanalysed the timing, duration, and magnitude of this event. We find that nekton and eunekton were well established prior to the Devonian and did not diversify dramatically during any Palaeozoic interval. Relative nektic diversity and occurrences decreased rather than increased during the Devonian. Eunektic diversity and occurrences increased throughout the Palaeozoic, but this rise was protracted and cannot be attributed to any single interval. Our new data indicate that the metazoan colonization of the water column was considerably more complex and gradual than previously understood.