The present report concerns energy-efficient seawater desalination systems utilizing either submarine or underground reverse osmosis units at suitable operative depths. The novel desalination schemes achieve remarkable energy efficiency through the free exploitation of the hydrostatic pressure of seawater and demonstrably require much less electric energy than any conventional (ground-based) desalination plant of comparable fresh water production capacity. The main operative features of the proposed submarine and underground seawater desalination technologies appear to promise great technico-economical potentials and advantageous couplings with renewable energy sources.