Doraemon: Nobita and the Castle of the Undersea Devil (1983)

Nobita and his friends under the waters of the Pacific Ocean. Gian and Suneo take Doraemon's underwater vehicle and travel through the Atlantic Ocean, trying to find a treasure ship. Along the way, they discover that the environment gun that Doraemon used to protect them is running out of energy leaving them vulnerable to the sea. Fortunately, they are rescued by some marine creatures.