A controversial proposal to build what would be the largest desalination plant in the nation along the Huntington Beach coastline was approved early today after months of raucous debate. The Huntington Beach City Council voted 4 to 3 to approve permits for Poseidon Resources Corp. to build a $250-million desalination facility next to the AES power station on Pacific Coast Highway at the city's southern edge. The desalination plant would produce as much as 50 million gallons of fresh water daily by tapping ocean water already pumped into the power station to cool the huge electrical facility. The plant still...