U-434 is a large Soviet submarine moored near Hamburg's Fischmarkt and opened as a museum. The cramped interior gives a direct sense of submarine life: narrow bunks, pipes, machinery, and passageways built for function rather than comfort.
It is an easy add-on to a walk through St Pauli, the harbourfront, or the Sunday fish market. Guided visits are the best way to understand what you are looking at inside.