And if any of you weren't already aware, it's the same u-boat from Das Boot, the film that came out the same year, 1981.
The outdoor mock-up was basically a shell propelled with a small engine, and stationed in La Rochelle, France and has a history of its own. One morning the production crew walked out to where they kept it afloat and found it missing. Someone had forgotten to inform the crew that an American filmmaker had rented the mock-up for his own film shooting in the area. This filmmaker was Steven Spielberg and the film he was shooting was Raiders of the Lost Ark.