I served on US diesel boats that did what were called, "arctic runs". Of course, the US boats were much more habitable, and larger, than the u-boats. But being up north was not at all comfortable. The in-line heaters were not very effective; the inside bulkheads were constantly damp with condensation, the engine rooms were cold when running but warm when submerged, the topside watches were terribly cold and wet. The u-boat sailors had to be tougher than me.

A bit of odd info: I stood watches in the forward room, which had deep bilges in the forward end. When snorkeling, the intake valve would often shut and cause rapid drops in pressure. This would cause a fog to form in the bilges, which would immediately disappear when the vale opened. Kind of like watching special effects in a movie based in "foggy London town".