In Raiders of the Lost Ark, the German submarine base where Indiana Jones boards the submarine is La Rochelle.
The sequence occurs after Indiana escapes the Well of the Souls and pursues the Nazi convoy carrying the Ark of the Covenant. He hides on the exterior of a submarine and reaches the German submarine base at La Rochelle, on the Atlantic coast of France. The location gives the pursuit a maritime setting before the Nazis transport the Ark onward.
La Rochelle was an important German naval base during the Second World War, especially for U-boats. Raiders uses the real wartime location as part of its fast-moving adventure plot, although the film’s action is fictionalized.
A common mix-up is to name Cairo, because much of the Ark storyline takes place in Egypt. Cairo is where Indiana reunites with Marion and works with Sallah; it is not the submarine-base location. The film’s fictional events are set in 1936.