Illustratory artwork of Sevii Islands on the client's mini-map
The Sevii archipelago is a collection of seven islands in the Kanto region and are completely optional. To access them you must speak with the Sevii Sailor in Vermilion City, you must have an active membership and be Kanto Champion.
At first, you will only be able to travel to One Island. By completing the quest there you will unlock the next island, and the same process unlocks the rest of the islands.