In this paper, we analyze the evaporation of a seawater film by mixed convection of humid air. The flown plate is heated and the second plate is dry and can exchanges heat with the environment, be isothermal or adiabatic. Using adequate approximations, we build up a nonlinear form of the Navier–Stokes equations, which is specific for the boundary layer case. We take into account the variation of the salt concentration because of the phase change along the vertical channel. Consequently, to find a solution for combined heat and mass transfers through the channel, we use a numerical finite difference method. The effect of salinity on transfer is taken into account. We found that adding salt to freshwater economizes energy to enhance the film temperature, and the latent heat flux is lower. In addition, we show that the influence of film matter quantity is clearer for saltwater when compared with freshwater case. However, we demonstrate that the effect of the film temperature at the entry and supply heating variations reminds constantly if we compare saltwater and freshwater.