Submarines aren't built with armor to face and survive direct gun fire from any helicopter. In reality, the submarine would be too damaged to submerge again.
Panzerfaust 3 anti-tank weapon isn't a MANPAD portable surface-to-air missiles as depicted in the movie.
The "Iranian" ship is actually an Independence Class Littoral Combat Ship in service with the US Navy, and the helicopter that takes off from said ship is a Chinese Z-10 Attack Helicopter.
The missile targeting France was launched from the Pacific Ocean, near Kamchatka. According to the movie, it flies above Russia, then Eastern Europe. With Earth being round, it would have most certainly flown above the North Pole.
In the introduction sequence at around 17 min, a helicopter takes off from a frigate. The submarine then rises to the surface and the commander shoots the helicopter with a rocket launcher. However, the frigate has apparently disappeared and we don't see it anymore, whereas it should still be around and be a potential threat to the submarine.
Conning tower of Titan destroyed by torpedo, but the main hull was intact, as filmed. So most of the crew would still be alive. Couldn't they then blow the tanks and surface? Same for the Missile sub. That torpedo did not hit the ship. It just had impact from detonating nearby. Hull was intact. Yes, some fire and air pollution. But they could have at least surfaced.
Omar Sy (D'Orsy) uses the US military salute, with the palm of the hand down; instead of the French military salute, with the palm facing outward.
The captain holds the weapon to his stomach while ordering the crew mate to shoot the lock with his rifle. No one in his right mind would stand behind a rifle target to do this.