A hydro-pneumatic energy storage system for deep sea water (DSW) is described. The system includes a floating support structure including a floating support platform, and a floating air chamber mounted on the floating support platform. The floating air chamber is configured for holding compressed air. The system also includes a sea-bottom mounted structure including a sea-bottom accumulator chamber configured for holding the compressed air and the DSW to store the DSW under pressure of the compressed air, and an air umbilical pneumatically interconnecting the floating air chamber with the sea-bottom accumulator chamber.