I t is appropriate that the center of Paris particularly that section gracing the Seine around Ile de la Cité and Ile Saint-Louis — features some of the city's oldest and most majestic historic monuments. For the past half-century, perhaps one of the most relaxing and expedient ways to view these sights in all their grandeur has been from the glass-covered decks of the Bateaux-Mouches, those unique long-boats leisurely plying the Seine, from whose vantage point millions of tourists have acquired their considerable appreciation for all that Paris has to offer. On some days, the sheer volume of this boat traffic resembles an invasion of a different kind, albeit one of camera shutters and tourists gazing in wonderment.