The day of reckoning is drawing near for Huntington Beach’s long-planned desalination plant, which would help quench Orange County’s thirst with sea water and free up imported water for the rest of the Southern California. Twenty years and $50 million into the process, officials with plant purveyor Poseidon are optimistic they will get their final two permits — possibly by year’s end. They tout the project as a drought-proof source of water that will provide a stable supplement to the more volatile groundwater and imported sources in a future filled with aquatic uncertainties.