Abstract: Subaqueous dunes hold significant research value in sedimentary geomorphology and marine engineering. Based on two phases of multibeam bathymetric surveys conducted in the western offshore region of Hainan Island, this study systematically examines the geometric characteristics and dynamic evolutionary mechanisms of subaqueous dunes. The results show that dune formation is governed by multiple interacting factors, including hydrodynamics, sediment transport processes, and water depth, with tidal currents serving as the primary driving force for dune initiation and growth. Dune evolution is jointly modulated by the interplay between bedload transport and suspended sediment concentration. The lag effect of suspended sediments represents a key mechanism contributing to dune development, while dune morphological stability is constrained by suspended sediments as well as by limitations imposed by water depth. Specifically, crest erosion suppresses further increases in dune height, whereas water depth regulates the critical scale of dune growth. Subaqueous crescentic dune height plays a dominant role in controlling morphological stability and hydrodynamic response, whereas dune width exhibits strong linear relationships with both height and wavelength, highlighting its integrative role in linking vertical and horizontal morphological development. The growth of crescentic dunes proceeds through four successive stages: initiation, chasing, merging, and splitting. Throughout this evolutionary sequence, sediment supply plays a crucial role in regulating dune wavelength and shaping the overall morphology. Correspondingly, dune geomorphology evolves sequentially from crescentic to curved, bifurcated, and ultimately linear forms. This study offers theoretical support and practical insights for understanding seabed geomorphic evolution, regional sediment dynamics, and marine engineering applications.