Reigo: King of the Sea Monsters (2005) on Netflix

Plot summary

Under Captain Yamagami, the battleship Yamato is ordered to the Truk Islands in September 1942 to join the Combined Fleet. Among the crew is the young Ensign Takeshi Kaido, who left for the war without telling Chie Kojima, the sweetheart of his childhood, what he felt for her, and who carries her photograph in his coat pocket. At Truk, a married officer brings an island woman named Momoka aboard the ship, and her elderly grandfather, who comes with her, tells him a tale of the man-sized, carnivorous Bonefish in these waters and of a greater menace behind them, Reigo, the Hell King of the Seas. The next night a lookout reports a huge shape barely breaking the surface, and the Yamato fires on what the crew takes for an enemy submarine.