Maritiman is the world's largest floating ship museum — nineteen historic vessels moored along the Göta älv river in central Gothenburg, all open to walk through. The collection includes a destroyer, a submarine (the Nordkaparen, a Swedish Navy vessel from 1962), a lightship, a firefighting vessel, and various tugboats and smaller craft. You can walk the decks, descend into the engine rooms, and in some cases sleep aboard overnight (a few vessels are fitted for this).
The submarine is the highlight for most visitors: cramped, low-ceilinged, and lit by the original instruments, it gives a visceral sense of what life underwater meant for the crew. The destroyer, by contrast, is large enough that exploring it takes most of an hour.
Maritiman is moored immediately in front of the city's Stenpiren ferry terminal, making it easy to combine with a trip to the archipelago. Open from April through September; hours are reduced in spring and autumn.