The lost heir - Tui Sutherland

The Wings of Fire saga continues with a thrilling underwater adventure--and
a mystery that will change everything!
The lost heir to the SeaWing throne is going home at last.
She can't believe it's finally happening. Tsunami and her fellow
dragonets of destiny are journeying under the water to the great
SeaWing Kingdom. Stolen as an egg from the royal hatchery, Tsunami
is eager to meet her future subjects and reunite with her mother,
Queen Coral.
But Tsunami's triumphant return doesn't go quite the way she imagined.
Queen Coral welcomes her with open wings, but a mysterious
assassin has been killing off the queen's heirs for years, and Tsunami
may be the next target. The dragonets came to the SeaWings
for protection, but this ocean hides secrets, betrayal--and perhaps
even death.