Minefield

Lt. Burke says the incoming torpedo is a VA-111 and has a warhead strong enough to set off the entire mine field. The VA-111 is a torpedo developed by the Soviet Union in the 1970's, it is a supercavitating long-range torpedo capable of extremely high speed. The nose cone of the torpedo has a special turbine that acts a bit like a jet engine, it is launched at a speed of 50 knots and as the torpedo accelerates the turbine spins and pulls water inside the nose where it reacts with the gas in the torpedo's engine exhaust causing a large bubble to form around the torpedo, this greatly reduces the torpedo's drag in the water allowing it to reach speeds of up to 200 knots, though it requires time and distance to reach those speeds. It is capable of carrying a nuclear warhead with a yield of 150 kilotons but typically is armed with a 460 pound conventional warhead with an estimated blast yield of 1,000 lbs of TNT. By comparison the Mark 54 torpedoes carried by the Nathan James have a top speed of 40 knots and carry a 97 pound warhead with a yield of 238 lbs of TNT.

When the sonar officer detects the incoming torpedo Captain Chandler says "there has to be a ship out there, find it". An experienced Naval officer like Chandler knows that there are multiple ways a torpedo can be launched, while the two most common are from a ship or submarine, torpedoes can also be dropped from helicopters, high altitude bombers and even fired from land based launchers. They are right next to an island so he should have at least considered the possibility that the torpedo was launched from land.