South Costa and Kosterhavet National Park

On South Koster - one of the islands in the Koster archipelago off the coast of Strömstad - lies Kosterhavet National Park. It's Sweden's first marine national park and there's plenty to explore underwater. See, for example, football sponges, arm-foots, pipefish and cold-water corals. There are local ferry departures from Strömstad that will take you out to the Koster archipelago.