Waking the Deep by P. Jameson

Cursed to reside in Sapphire Lake until he finds love was never how the warrior, Mansen, pictured his fate. Having a shimmering tail instead of legs wasn’t on his bucket list either. For ages, he’s searched for her, the one who can free him from the water, but came up empty handed. Now, he’s resigned to living as a merman and making his three days a month on land count… in carnal ways. Until he finds a female in his lake that needs rescuing.
Doe doesn’t know her real name. Or where she came from or who she was before the overbearing man fished her from the lake where she was drowning. She only remembers the fear that sent her into the lake in the first place. Fear she can’t explain, yet it’s the only thing that feels real. It and the strange, exciting connection to the man who saved her. But he has secrets she might not be ready to accept and her secrets may be even bigger.
Together Mansen and Doe will have to untangle the mystery surrounding them and Sapphire Lake if they ever want to live free. But are they ready for what they’re about to face. Thrills, spills, and… tails?