Koster consists of Nord-Koster (North Koster) and Syd-Koster (South Koster) island, plus many small islands. In Swedish, Koster marine national park is called Kosterhavet marin nationalpark.
You can get to Koster national park by taking the Koster ferry (Kosterfärjan) from Strömstad, Sweden, which lies along the E6. Koster lies about 2.5 hours north of Göteborg or 2.5 hours south of Oslo, Norway.
Keep in mind that there are no cars allowed inside Kosterhavet national park, but you can rent bicycles in the harbor when you arrive on Syd-Koster.
There's a maze of small islands, some of them so close to each other that boats can't pass through them. This in turn leaves the animals at Kosterhavet national park in peace and they love it. There are 300 animal species living here that are unique in Swedish waters. The Koster islands also are home to traditional inshore fishery, basking sharks, harbor seals, and Sweden’s only cold-water coral reef.
Camping is allowed at Koster national park, by the way. And if you like lighthouses, you can find one at Ursholmen, the westernmost inhabited island in Sweden.