Heterodyne detection is an efficient approach for weak signal detection in underwater optical communication. Based on the generalized Huygens-Fresnel diffraction principle, a coherent detection mathematical model for the propagation of partially coherent Gaussian Schell beams in ocean turbulence is established. The model considers the transmission characteristics of partially coherent Gaussian Schell beams, and an analytical expression for the heterodyne efficiency of partially coherent Gaussian Schell beams in ocean turbulence environments is derived. The effects of the light source parameters, ocean turbulence parameters, transmission distance, and detector aperture on the heterodyne efficiency are numerically investigated. The research results show that the effects of the light source parameters on the heterodyne efficiency are negligible when compared with that of the ocean turbulence. Moreover, increasing the detector aperture does not improve the heterodyne efficiency, although selecting an appropriate detector aperture can effectively suppress the ocean turbulence effect in an environment with a specific ocean turbulence intensity. Hence, the conclusions of this study provide theoretical guidance for coherent detection systems in underwater wireless optical communication.