Within shallow coastal areas, the waters of open ocean dissipative sandy beaches can be differentiated into the surf zone, where wave action generates turbulent conditions, and the subtidal zone, characterized by higher depth and calmer conditions. These differences in hydrodynamic conditions could provide specific habitats for fish populations. This study conducted at Monte Hermoso beach, Argentina (38°59’ S, 61°18’ W), analyzed the spatial–temporal variation in fish assemblage diversity in the surf zone (SZ) and the adjacent shallow subtidal zone (SSZ). Samples were simultaneously collected in both zones during low tide using a 10 mm (knot-to-knot) net, dragged for 300 m parallel to the shoreline. In the SZ, hand-towed trawls were conducted at a depth of 1.2 m, while in the SSZ, samples were collected at a depth of 5 m aboard a vessel at a constant speed of 4 km/h. Overall, 210 individuals belonging to 7 species were caught in the SZ and 1864 individuals belonging to 27 species in the SSZ. According to the graphical analyses based on the Olmstead–Tukey test (discriminant criterion = arithmetic means), Odontesthes argentinensis and Ramnogaster arcuata were dominant in the SZ, while R. arcuata, Cynoscion guatucupa, and Anchoa marinii dominated in the SSZ. Richness and Shannon and Simpson’s diversity, evaluated through Hill numbers, were higher in the SSZ during all seasons and showed no differences in winter. A high dissimilarity value was registered between the zones (β_sor = 0.64), and the nestedness component (β_nes = 0.50) was higher than the turnover (β_sim = 0.14). This study represents the first record of shallow subtidal fish assemblages in Argentina and reveals that the wave action in the SZnegatively affects the richness and abundance of the fish assemblage. However, certain species, such as R. arcuata, exhibit remarkable plasticity in adapting to both environments.