Physical Oceanography is the branch of marine sciences concerned with understanding the oceans as a geophysical fluid system, focusing on the processes that govern the spatial organisation and temporal variability of momentum, heat, and mass across a wide range of scales. Knowledge in Physical Oceanography is produced through a combination of observations, theory, numerical models, and statistical inference; yet its epistemological structure remains largely implicit. This paper develops a framework to clarify how such knowledge is generated, validated, and constrained by the ocean’s vast spatial extent, continuous variability, and multiscale dynamics. We propose a two-dimensional epistemic map that classifies knowledge claims by their aim and representational form, providing a structured vocabulary for interpreting the diversity of oceanographic research. We illustrate the map with an exploratory classification of scientific synopses, and use it to read recent tendencies in the discipline. We further examine the epistemic status of oceanic numerical models and reanalysis systems, and present a case study of wind-wave spectral representation that exposes the underlying assumptions and limits of common practices. Building on these elements, we propose a diagnostic reporting template aimed at making explicit the assumptions, validation criteria, and epistemic payoff of individual studies. By rendering the epistemic structure of a result legible, the framework supports clearer interpretation, comparison, and communication of physical-oceanographic knowledge, both within the discipline and across the wider Earth-system sciences.