Emily Windsnap and the Castle in the Mist by Liz Kessler

Main Character: 12-year-old Emily Windsnap, half-mermaid, half-human
Location: the Ocean
Time period: Contemporary
Genre: J Fiction, Fantasy, Mermaids
Series: Emily Windsnap #3

Emily's teacher is terribly excited--King Neptune is coming on a royal visit! All the mermaids are sent out to find pretty stones, gems, gold, and other adornments to present to the king when he arrives. Emily is mostly hoping to escape his notice, since he is still not happy with her parents and their merman/human alliance. Unfortunately Emily's offering turns out to be a significant diamond ring--not only for it's size and beauty, but for it's personal meaning. It turns out that Neptune himself once fell in love with a human woman. They exchanged rings, one diamond and one pearl. But she disappeared and Neptune believed that she abandoned him which led him to forbid any such alliances again.

The sight of the diamond ring causes Neptune to go into a rage and send Emily's boat out into the ocean, far away from anything familiar. They are lost in a cloud of mist and Emily knows that the only way she can get them back home is to get to a castle on the island they can see far away in the mist.

I managed to read this series out of order, but even without having read the first book I was able to get into this one right away. Now I'm going to have to seek out the rest of the series.