War Serpents

It is 1944, the peak of warfare on the Pacific Theatre. You are Gregory Watson, an officer stationed on the brig of the USS Missouri. Your ship encountered some Japanese soldiers shipwrecked on a desert island. They were taken to the brig, and they have been claiming sea serpents destroyed their ship, the Shinjitsu. Everyone is laughing it off as insanity, including you, but then, when you were smoking on deck, you saw a strange, extremely large creature swimming in the oceans below.