The ETravelSim Sweden eSIM works at the main Stockholm Archipelago islands including Vaxholm, Sandhamn, Grinda, and the Waxholmsbolaget ferry routes near the mainland. Signal softens or drops on smaller outer islands like Utö's remote eastern shore and on the open-water sections between islands. The west coast islands of Marstrand, Käringön, and Smögen have generally reliable mainland coverage. Save offline maps and ferry timetables before boat days because connectivity isn't guaranteed mid-crossing.