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.