A historic boat parade downthe River Seine launched the Paris Olympics with spectacular French flair Friday, as the City of Light welcomed the world’s greatest athletes for a sporting extravaganza. Braving torrential rain, some 300,000 people lined the river banks to cheer on the armada carrying competitors past the city’s iconic sights: the Eiffel Tower bearing the five Olympic rings, the Louvre and Notre Dame Cathedral. The wildly ambitious display was the first time the Olympic opening ceremony has been staged outside the main stadium, making it the biggest-ever launch for the “Greatest Sho…