The dragon god of the sea in Japanese myth is Ryujin.
Ryujin, written 龍神 and also rendered Ryūjin, is a powerful sea deity associated with the ocean, rain, thunder, and the tides. Mythic accounts place his palace, Ryūgū-jō, beneath the sea, where he controlled the waters with magical tide jewels.
Ryujin appears in several famous legends. He is the father of Princess Otohime, who welcomes the fisherman Urashima Tarō to the Dragon Palace. In another story, the hero Tawara Tōda helps defeat a giant centipede threatening Ryujin’s realm. Ryujin is also connected with prayers for rain, fishing, and agricultural fertility.
A common mix-up is Watatsumi, another important Japanese sea deity. Some traditions treat Watatsumi as another name or counterpart of Ryujin, but Ryujin is the standard answer when the question specifically asks for the dragon god of the sea.