There were rungs on the conning tower, so ropes wouldn't be needed. Seems like ropes would make you pretty vulnerable to small arms gunfire where as using the rungs would be quicker. Must movies and films about U-Boats prior to
Das Boot had a lot of screwy detail.
The Enemy Below in particular. I don't think the deck hatches on a U-Boat were meant to be used outside of port given a U-Boat's low freeboard.
This must have been the "Judgment Night" Twilight Zone episode with Nehemiah Persoff.