Correct me if I'm wrong, but I think it's called "Tidanakim." And N'ak was half payanak(waterdragon), half human, not mermaid. Your description of the movie does sound like this movie.
Short Summary:
N'ak mom was human and her dad was the dragon king under the sea. One day he came up to the surface and got injured from fighting with the big bird guy. N'ak mom happened to find him and took him back to her palace and cured him. They then fell in love. He then left because he was not well cured so he has to returned back to the sea to meditate to cure his wound. Later, N'ak mom got pregnant, and she was accused of sleeping around and got pregnant without a father. She was kicked out of the palace after given birth.
I don't really remember how she lost her baby....but anywho..after she lost her baby she was kidnapped by this lobster guy and he tried to force her to become his wife, but her husband (the waterdragon) came to saved her in time. She told him about their daughter and they went out searching for her. But they couldn't. They decided to "jum sin pawana" (the word that means they have to meditate for years) for their sins.
Blah, blah.........Years pasted by their daughter(N'ak) grew up. He has magical powers as always and can take care of herself. She came across P'ak palace once more(she and P'ak met already when they were little and they became close, and he was blind and she cured him).
Blah, blah.....They got married and she became pregnant..N'ake parents came up looking for her and they found her.
Okay that's all I can remember because this lakorn doesn't have an ending.