It has been suggested that wavepower can be used for seawater desalination using unsteady incompressible duct flow to create the hydrostatic pressure for reverse osmosis [1]. This approach requires further investigation in a number of areas: the non-linear relationship between reverse osmosis recovery rate and output of fresh water for the proposed system; determination of the optimum recovery rate; pretreatment costs for the optimum recovery rate; and operating costs in comparison to conventional desalination plants of varying sizes, recovery rates, seawater types and intake. The reason for the non-linear relationship between recovery rate and output is explained, and the optimum recovery rate for Mediterranean seawater is determined. Pretreatment costs for this recovery rate and operating pressure are determined, Operating costs for the proposed system are estimated and compared with those for open sea intake and seawell intake desalination plants in the Red Sea, the Arabian Gulf, the Mediterranean Sea, oceanic seawater and the Caribbean Sea.