Poseidon Water hopes to help quench Orange County’s thirst, but first the company’s proposed desalination project must slake a thirst of its own. That’s why Poseidon has long eyed a coastal power plant that has, for more than a half-century, sucked up seawater to cool its massive generators. The AES Huntington Beach Generating Station’s giant smokestacks and steam boilers will be gone in a few years, replaced under state orders by a smaller plant that uses air, rather than the ocean, to keep from overheating.