Indonesia has a lot of islands. Beside the big islands like Java, Sumatra, Borneo, Sulawesi, and Irian, there are a lot of small-beautiful islands. If you visit Jakarta, the capital of Indonesia, you should go to the north. There are Thousand Islands (or Kepulauan Seribu). You just have to take a boat, one until two hours, the you will have already reached island like Pramuka Island, Tidung Island, etc with clean-white sand and you can go snorkling too.
This photo was when I went there with my family. We just swimming in the beach until the sunset.