Seeking the Deep by P. Jameson

Welcome to Aurora Falls. Home of platter-sized pancakes, the Northern Lights, and… a lake full of finned immortal warriors in search of their mates. This mountain getaway is full of sexy shimmering secrets just waiting to be revealed…
Jase McCarthy has lost a mermaid. Not too many men in the world can say that and mean it. Aurora Falls has been his home for his whole life, but he never believed the rumors about Sapphire Lake. Not until one night with a rough-talking… rough-riding… beauty he can’t get out of his head. Never should’ve mentioned that strange tattoo he’d noticed on the small of her back while she was pretending to be a cowgirl around his waist. It means something, and he’s going to find out what. But first, he’ll have to find HER.
Vada is fine with the fin she wears for most of her life. Three days a month out of the water is enough. Less time on land means less time to hurt people. Less bodies to bury. Less hearts to accidentally break. Besides, she’s not looking for a mate like the others of her kind are. So when she finds hers alone in a bar, looking like a wet dream come true… she loses her senses for a night. It’s okay. It’s fine. The water hides her secrets, and she’s got a big one Jase can never find out.