Nereid

The story starts off like any other sea-princess tale: girl goes on a diplomatic journey. Girl is a princess. Girl is threatened by pirates.
That's where the similarity ends.
Tessa Gray, crown princess of the Novae Empire, is on a diplomatic mission to the neighboring kingdom of Inferos, to a conference that could stop the brewing war between the two superpowers---or ignite it. And if that wasn't nerve-wracking enough, the one of the most ruthless pirates who's ever sailed the waters of Inferos-Novae seas, has set his sights on the princess...and he's willing to hunt the royal embassy to the ends of the earth to get her, and to kill whoever gets in his way.
And Tessa---well, she'd rather die than become the Magister's tool. The princess dresses as a man to prevent the Magister's men from following her into the unforgiving waes and throws herself over the side, knowing only her death will deter him.
Except she doesn't die. She wakes up in a cove on an island, the sun shining through a hole in the top of the cave, lying on the sands.
And sitting by her side are two men---boys, really, but they're far from human.
They're mer---but they aren't.
They're Nereids, the race of the sea, the people of the arts...and of the war. But slowly, Tessa finds herself drawn to the Nereids, to the strange seafolk and their beautiful world---but just when things seem to settle into some form of "normal", they discover that not only has war sparked between Novae and Inferos, but that the Magister has begun to wage war on the sea.
Things just got a whole lot more complicated for the princess and the Nereids who saved her.