A system for desalinating water is disclosed. The system comprises a subsea reverse osmosis unit located beneath the surface of a body of water, a first liquid column comprising seawater, a second liquid column comprising desalinated water with a salinity less than seawater, and a brine discharge outlet. Due to the difference in density between the seawater and the desalinated water, the gravitational hydrostatic pressure of the first liquid column may be greater than the gravitational hydrostatic pressure of the second liquid column. At least a portion of the pressure difference for reverse osmosis desalination may be provided by the difference in gravitational hydrostatic pressure between the first liquid column and the second liquid column. A significant reduction in desalination energy consumption may be enabled by discharging the brine at an elevation lower than the maximum elevation of the first liquid column or the second liquid column.