Intro: The story starts in a strange place. You drop us down into the middle of a a big event and it's kind of awkward at first.
Ooh, mermaids! I have sufficient lack of mermaid stories in my life. =)
He didn’t know how he could bare to send her away, knowing she would grow up believing another merman was her father.”
Bare is to "bare skin" or something. Should be "bear". Also, ditch the quotation mark at the end.
twinkling in the artificial light
Where is the artificial light coming from?
He had been trained as a warrior and advisor, the mermaids raised the children
Comma should be a semi-colon.
Wait - that woman that came with the ambulance crew - how does she know her father is dead? You mention that she stays with Ariana, so she really has no way of knowing the actual state of her father, and then she begins telling her he's in heaven. You need to fix this, because as it is, it's hasty and a little morbid.
With that, I also have to mention that your plot pace is much too fast. You need to slow it down, add more descriptions, more emotions and thoughts so we don't feel so thrown into the storyline.
Characterization is good. I can really get a sense of the King's anguish over his daughter. Poor father. I think you should continue because I'd really like to find out some things, like how Ariana lost her tail, and if Dave was Darien.
Thanks for the read! I hope I helped. PM me if you have any questions. =)