Gary, Doug, et al:

It was a post-war U.S. Navy fleet boat with the snorchel and streamline aluminum sail configuration.

In the scene when the 'escapees" are climbing aboard the sub, you can see the brass salvage air system plates attached to the fir deck planking, as they head toward the sail access door to get below.

Now keep rolling the film (video tape) and you'll note that the OOD
(a three-striper) on the bridge is situated just aft of the retractable Plexiglas "bubble" and the TBT (Target Bearing Transmitter) is located centerline behind him.

Look at the round edge of the steel superstructure plating on the starboard side of the sail. U-boats did not have that type of construction until the Type XXI boats came along.

Now, as the boat dives, (and the caption reads 'You dogs") you'll
see the top of the sail clearly. The radar antenna is raised, one of the 'scopes is raised, a communication antenna is raised near the aft end of the sail and the snorchel and snorchel-whip antenna is visible. But the streaming commissioning pennant located just under the snorchel on the trailing-edge of the sail is the real clincher.

Keep rolling the movie until you see the shot of the sub underwater
sneaking away........ Now that's some real weird submarine from ABACUS PRODUCTIONS!

Hope this answers your question,

ROBERT M.