The turbines for underwater applications can use abundant water as the cooling medium. However, the use of liquid water is associated with the multi-phase and multi-physics problems and presents additional losses. This paper proposes a numerical coupling method and corresponding loss breakdown approach to analyze the performance of this particular turbine cooling problem at different water mass flow rates. The results show that the cooling water induces additional loss, while the influence of the resulted structural deformation on turbine efficiency is comparatively small. Through numerical loss breakdown, it is found that the exit kinetic energy loss is significantly decreased, but other losses are increased correspondingly due to the cooling water. By examining the turbine stress and temperature distribution, the small water mass flow rate of 0.02 kg/s is sufficient to avoid damage to the disc, however, additional cooling is necessary to minimize the plastic region at the turbine blades. The suitable loss model for the turbine with cooling water is established and a relative difference less than 3 % is attained by comparing mean-line and numerical results. This work provides new insight into the analysis method and characteristics of the partial admission axial turbine with cooling water.