Moby Dick

The film begins in November 20, 1969, 50 miles off Soviet waters. When the USS Acushnet dives under the ice, a young Ahab listens for enemy subs when suddenly he detects an unknown target. When the captain listens, he hears nothing, but Ahab insists in the presence of an emptiness. The target dives into a trench, but the captain abandons his search in favor of photographing the target. The target attacks the submarine as Ahab hears a roar. The sub is brought to the icy surface, and the target identifies itself as a gigantic, white, whale-like creature. Ahab survives, but loses his leg to the beast when it hauls the other half of the sub back underwater.
In the present day, Dr. Michelle Herman (RenÃ©e O'Connor) and her assistant Pip (Derrick Scott) test a whale song generator when the USS Pequod surfaces behind them and Lieutenant Commander Starbuck (Adam Grimes), the executive officer, persuades them to come aboard. In the sub, Starbuck tells them about several attacks in which eyewitnesses all report seeing an enormous whale. Although Michelle explains that the whale song generator needs a recorded whale vocalization, Captain Ahab (Barry Bostwick) comes to the deck and gives her the recording he took of Moby Dick back in 1969. Although Michelle disagrees on joining a Navy sub with the intent of killing an animal, she has no choice.
In San Diego, Captain Boomer (another survivor of the attack of '69) is told by his superiors of suspicious activity revolving around the Pequod. He is assigned to investigate, and thanks to a survivor from a recent attack by the White Whale, he comes to the conclusion that Ahab is on the hunt for Moby Dick.
Meanwhile, the USS Essex is searching for the Pequod off Hawaii. When they go to active sonar, it attracts the attention of the wandering Moby Dick. The Essex engages what appears to be the sub they were searching for, but realizes too late that their adversary is biological just before the sub is destroyed by a torpedo it shot. Later the Pequod comes to their location with no sign of the whale, but encounter the corpses of Essex crew. Rousing his crew with a speech, Ahab moves on to search for the beast.
A helicopter in search of the Pequod encounters it while the sub is following a 600-foot target. As the helicopter engages them, the sub fires a nuke at the unknown target, but the confused helicopter crew tell them they shot a school of giant squid just before they are swallowed alive by Moby Dick.
The whale then attacks the S.S. Rachel, a cruise liner, when the Pequod intervenes with Michelle's whalesong generator. This, however, causes the whale to attack them, subsequently destroying a fin on the Pequod. The sub fires a harpoon made from the Acushnet's hull on top of Moby Dick's eye, which forces him to dive deeper, dragging the Pequod with it. As the water pressure begins to damage the hull, the line snaps and Starbuck forces the ship to surface.
Moby-Dick surfaces too, and the Pequod, along with the help of Boomer in a helicopter, forces the whale into an atoll. The sub gets trapped in shallow water, and three boats are sent out to face the whale with guns and Ahab's harpoon. Moby-Dick destroys two of the boats and forces the survivors onto the island's shores. The whale attacks them again, resulting in the death of Queequeg (Michael Teh). Ahab takes the last boat and fires his harpoon at the whale's other eye. Moby Dick destroys the boat, killing Ahab. The remaining crew of the Pequod, including Starbuck, and Pip, follow Ahab's orders and fire nukes at the island. Moby Dick dodges the nukes and crushes the Pequod just as the island explodes. The film ends with the White Whale surviving to wreak havoc another day, and with Michelle swimming to the surface just as a rescue helicopter arrives.

Question: When does the sub get trapped?
Answer: Shallow water