Under Siege

The battleship USSÂ MissouriÂ (BB-63) arrives at Pearl Harbor, where George H. W. Bush announces that the ship will be decommissioned in California, making the trip her final voyage. Casey Ryback, a Chief Petty Officer assigned as a cook, prepares meals in celebration of the birthday of Captain Adams, against the orders of Commander Krill, who is having food and entertainment brought by helicopter.
Krill and other officers provoke a brawl with Ryback. Unable to imprison Ryback in the brig without clearance from the captain, Krill detains Ryback in a freezer and places a Marine, Private Nash, on guard. A helicopter lands on the ship's deck with a musical band, along with Playmate Jordan Tate and a group of caterers who are really a band of mercenaries led by ex-CIA operative William "Bill" Strannix.
Strannix' forces seize control of the ship with Krill's help. Several officers are killed, including Adams. Ryback hears the gunshots and begs Nash to free him, but Nash refuses, thinking it is fireworks. The surviving ship's company are imprisoned in the forecastle, except for some stragglers in unsecured areas. Strannix intends to sell the ship's Tomahawks by unloading them onto a submarine he previously stole from North Korea. The CIA tried assassinating him prior to the events of the film. Strannix and his men take over the ship's weapon systems, shooting down a jet sent to investigate, and plan on covering their escape by using missiles to obliterate tracking systems in Pearl Harbor.
Krill realizes they forgot about Ryback and learns the kitchen is an unsecured area. Strannix sends two mercenaries to eliminate Ryback and Nash. Nash is killed, but Ryback eliminates the assassins and also creates a time bomb.
Ryback contacts Admiral Bates at the Pentagon on satellite phone, whereupon the Navy plans to send a SEAL team to retake the ship. Krill discovers Ryback is not just a Chief Petty Officer but a former Navy SEAL with extensive training in anti-terrorism tactics. To keep the missile-theft plan in place, Krill activates the fire suppression system in the forecastle, leaving the crew members to drown. The terrorists expect that Ryback will try to save his colleagues, and set up an ambush.
Ryback hears six sailors banging on pipes in Morse code and rescues them. Together, they overcome the ambush, shut off the water in the forecastle, and eliminate several terrorists. Ryback shuts down the Missouri's weapon systems to allow the incoming Navy SEALs to land. However, the submarine crew shoots down the helicopter carrying the Navy SEALs with shoulder-fired surface-to-air missiles. The Pentagon responds by ordering an air strike that will sink the Missouri. Strannix regains control of the ship's weapon systems and loads the Tomahawks onto the submarine. With the aid of a retired World War II Gunner's Mate, who was among the six sailors rescued earlier, Ryback uses the battleship's 16Â inch guns to sink the submarine, killing Krill and everyone on board.
Strannix suffers a major concussion from being in the proximity of the Missouri's guns as they are fired. He launches two nuclear-tipped Tomahawks towards Honolulu. As the sailors recapture the ship, Ryback finds his way into the control room, where he is surprised by Strannix, both of whom realize they know each other from prior covert experiences. Ryback disarms Strannix, and the two engage in a knife melee. Ryback gains the upper hand and stabs Strannix through the top of his head before ramming him into a computer monitor. Ryback takes the launch code disk needed to self-destruct the Tomahawk missiles. A fighter jet destroys one of the missiles, and the other is deactivated just in time; the Navy calls off its airstrike.
The remaining crew members are freed as the ship sails towards San Francisco harbor. Ryback is checked by a Navy doctor, kissing Jordan as the crew looks on and cheers. A funeral ceremony for Captain Adams is held on the deck of the Missouri, showing the Captain's casket with a flag draped over it and Ryback saluting in his formal dress uniform with full decorations.

Question: What did Nash think the gunshot sound was?
Answer: fireworks