German song/ballad 'Die schöne Lilofee' (first line: Es freit ein wilder Wassermann...):

A king's daughter marries a water spirit and lives with him under water. Homesick she comes to the land again to see normal humans. Her husband says she can live again at land but without her children. With the heaviest of heart she decides for her children and lives under water again. Now that's a song about underwater life!

Wolfgang