CANNES, France—The Cannes Yachting Festival, held during the second week of September in the French Riviera, is one of the largest boat shows in the world, drawing some 50,000 annual visitors to this small coastal city to gawk at massive, shimmering vessels whose price tags run well into the eight-figure range. Only this year, the dazzle was somewhat dulled. A few weeks before the show, the local government sent a letter to exhibitors: Because Europe is experiencing what could become its worst drought in 500 years, yacht companies were strictly prohibited from washing their boats using publicly accessible water. In the absence of rain, there would be more dirt than usual on display.

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