Abstract: The water-planing process of seaplanes involves coupling between aerodynamic and hydrodynamic forces and aircraft motion. Free-surface deformation, wave impact, and spray jointly affect planing performance and takeoff and landing safety. This paper reviews research on the mechanics of seaplane water planing through theoretical analysis, numerical simulation, and experimental investigation. First, hydrodynamic theories for planing and their applications to seaplanes are outlined, together with the relationship between theoretical analysis and numerical simulation. Numerical modeling methods and their applicability are then discussed through specific studies, with emphasis on wave-induced planing responses and seakeeping characteristics, configuration improvements, engineering analysis methods, and longitudinal stability. Finally, advances in tank and on-water testing are summarized, including scale effects and the extrapolation of model-scale results to full scale. Existing studies show that planing resistance, loads, and motion stability depend jointly on aircraft configuration, center-of-gravity location, speed, and wave conditions, and that the benefits of configuration changes vary with operating conditions. Further research is needed on aerodynamic–hydrodynamic coupling in complex waves, step ventilation and spray, longitudinal stability, and full-scale extrapolation. Combining numerical simulation with experimental validation can help establish the applicability of analysis methods and design solutions, providing a basis for seaplane performance assessment and design.