Some people laugh outright. Others raise their eyebrows or grimace. On the stone quays of the Seine, even the most passionate odes to the river’s charms tend to arrive at the same conclusion: Nope. “Swim in it? Me?” said retiree Patrice Desrousses, 64, recoiling ever so slightly as he paused in his promenade beside the storied waterway. “Oh, I think not.” Less than two weeks before Paris inaugurates its first Olympic Games in a century, the Seine — watery muse to poets and painters, backdrop to the city’s most majestic monuments, sighed over by generations of lovers — remains the designated op…