Nowadays, the energy demand is continuous increasing due to the growing population in the world. However, the major primary energy resources, fossil fuels, are inadequate. Thus, it is required to find another energy resources, especially the renewable ones, to meet the energy demand. One of prospective renewable energy resources is marine energy. In this paper, an ammonia-water zeotropic mixture is proposed to improve the ocean thermal energy conversion system's efficiency. Two scenarios of the ammonia-water zeotropic mixture are selected i.e. a mixture of 95% ammonia/ 5% water and a mixture of 97% ammonia/ 3%water. The ammonia-water zeotropic mixture working fluid provides much better efficiency than the pure ammonia working fluid, especially the mixture of 97% ammonia/ 3%water provides the best net efficiency of 4.71 %.