Sardinia has two distinct faces. One is wild, natural beauty. With some of the most gorgeous waters and unspoiled beaches in the Mediterranean, it’s no wonder that its northeastern coast, the Costa Smeralda, is a favorite spot for yachters. When I was there in June, we spent a morning exploring islands where the sandy beaches were completely deserted. (The waters truly do glisten green and blue like emeralds, from which the coast takes its name.) We had sheltered, shallow coves to ourselves, and...