The Naval Museum occupies the old dockyard at Stumholmen, just east of the city centre, and is one of the best maritime museums in Sweden. The collection covers Swedish naval history from the 17th century to the present day: ship models, figureheads, weapons, uniforms, and the actual submarine HMS Neptun, which you can board and walk through. The dockyard buildings themselves — rope-walk, sail loft, smithy — are part of the UNESCO World Heritage Site.
Marinmuseum
Sweden's naval museum with a walkable Cold War submarine