You can use the hydrostatic pressure (about 60bar) with a simple hydraulic intensifier to lift RO permeate from ~600m to ~100m depth, then finish the last stretch with a small, diver-serviceable booster. Basically you only need power to pump the final 100m which isn't bad.

Why not use the deep ocean hydrostatic pressure with a simple hydraulic intensifier to lift RO permeate from ~600m to ~100m depth, then finish the last stretch with a small, diver-serviceable booster? This removes deep rotating equipment, cuts energy dramatically versus full-depth pumping, and keeps maintenance simple and close to the surface.