The plant, to be built by Boston-based Poseidon Water, would produce 50 million gallons a day of drinking water, enough for about 400,000 people. Poseidon’s plant of the same size in Carlsbad is currently the largest desalination plant in the Western Hemisphere. The state lands lease, which runs through 2026, lets Poseidon use pipes from a coastal power plant to suck seawater into reverse osmosis pipes and spit back concentrated brine after the water is desalinated.