In the opening scene, the crew is on the bridge of the sub, looking out for ships and the enemy. The captain sees a Sunderland flying boat in his binoculars approach. The sub dives to avoid attack. Moments later the sub is attacked by 2 destroyers and sunk. Yet no one on the bridge on observation duty saw either of the destroyers in the area. The destroyers could not have been so far from the sub that they could attack moments after the Sunderland.
The boat was leaving at sunrise and into the sun. La Rochelle faces the west. The sun should have been behind the boat.