Super image, well done. Actually about 15% of female narwhals have these magnificent tusks, and not all males have them. Some even have two tusks! When you look at a narwhal, it doesn't look like a tooth growing out of the creature's lip. It looks like a horn growing from its head, so think the same for this narwhalmaid. animals.howstuffworks.com/mamm… has quite an in depth article.
Is it bad that all I could think of when I saw this deviation's title was; ♪ Narwhals, narwhals swimming in the ocean, causing a commotion 'coz they are so awesome! ♪ ...? (It's a weebl song fyi)
Also your description made me giggle XD I honestly wouldn't have even thought of it! I just looked at this and thought, Ooo! A narwhal mermaid! What a neat concept! I love her sassy expression; she's all "This is MY ocean. Don't make me stab you, bitch." Stab you with her... horn/tooth that apparently only... males have... oh who cares!!! She's a cool mermaid. End of story.