As we are here on a site dealing especially with german u-boats in wwII, i´ll focus on them.

In germany there are quiety a lot places to visit, when youre interested in u-boats:

in Kiel you´ll find the naval memorial:
[en.wikipedia.org]

just on the same place you can visit a original german u-boat, that saw action in wwII. you can see the u-boat directly from the top of the Naval Memorial, as it lies directly to its feet:
[en.wikipedia.org]
[de.wikipedia.org]

about 20 minutes of walking you will reach this place, where all names of german u-boat sailors are written on bronze tables boat by boat, which were lost in WWI and WWII.:
[de.wikipedia.org]

These 3 objects are reachable from Kiel with ferry boat. About 30 Minutes driving.
The ferry departs directly from Kiel harbour.

In Kiel there you can visit also the Kilian Bunker, a former u-boat bunker, which contains now a museum. Also, when your are lucky you can see some warships there. Last summer I saw the Gorch Fock and our new Type 212 u-boat.

In Cuxhaven-Altenbruch, also on the coast of germany, but this time the north sea coast you can visit this one:
[www.u-boot-net.de]
Its the world famous U-Bootmuseum, former U-Bootarchiv, founded by a former german u-boat veteran from wwII. There youll find so much uniforms, medals, paintings, photos etc....... Just a real and unique museum. But you have to make a visiting call. Very impressing.

Hope this helps a bit.

Best regards