In Over Her Head — Judi Fennell

HE LIVES UNDER THE SEA
Reel Tritone is the rebellious royal second son of the ruler of a vast undersea kingdom. A Merman born with legs instead of a tail, he's always been fascinated by humans, especially one young woman he once saw swimming near his family's reef...

SHE'S TERRIFIED OF THE OCEAN
Ever since the day she swam out too far and heard voices in the water, marina Owner Erica Peck won't go swimming for anything -- until she's forced into the water by a shady ex-boyfriend searching for stolen diamonds and is nearly eaten by a shark. Luckily Reel is nearby to save her, and he discovers she's the woman he's been searching for...

Before she knows it, Erica is off with Reel on a series of wild underwater adventures to retrieve stolen treasure, battle the mother of all sea monsters, and defy a death edict from the kingdom's high council. But even if they get out of their madcap mission alive, how will they Iii a way to stay together when she hates the water and he can't survive on land?