Vrnik is a small island off the bigger Korčula island in the Adriatic Sea. Vrnik has only a handful of houses and remnants from its past as a quarry for stones. There are two restaurants, but no shops, and no services. But it has great, clean, beautiful waters. The beaches are rocky but it is a pleasure to swim in them.
There's also plenty of natural seashore, in front of various house as well as around the island. You can (almost, there's one private residence) walk around the entire island, and swim anywhere. I really liked the east shores (pic above) for their quietness and lack of any houses. Easy access to water, too.