The performance of three combined systems using reverse osmosis to produce drinkable water and a Rankine cycle which produces mechanical power have been analyzed and compared. The RO subsystem incorporates a power recovery unit (a hydraulic turbine in the first two cases and a pressure exchange unit in the third case). The coupling between the RO and Rankine subsystems is only mechanical in the first case (the Rankine cycle provides mechanical power to the pumps of the RO subsystem), while in the two other cases the coupling is both mechanical and thermal (the heat rejected by the condenser of the Rankine cycle is transferred to the seawater). The minimum values of the Rankine cycle mass ratio for the three systems and the maximum operating temperature for the two last systems have been established. Energy and exergy efficiencies are also compared for identical entering parameters.