The desalination plant in Carlsbad is celebrating the production of 40-billion gallons of water since it opened three years ago. That’s enough water to fill the Empire State Building 145 times. Or a billion bath tubs. The “watermark” being celebrated Thursday with a gathering of officials, including former U.S. Senator Barbara Boxer, Carlsbad Mayor Matt Hall and Poseidon CEO Carlos Riva. All sharing a toast with a glass of fresh water produced by the plant.