Warrior’s Kiss: Mountain Mermaids by Victoria Flynn

Ivar is a Cursed One, a man doomed to live eternity beneath the waves by an angry, heartbroken enchantress, Tamsin. His only chance at freedom is to find his true love and convince her to take him as a mate. Only during the full moon may he rise from the depths of Sapphire Lake to find the woman destined to be his. There’s a catch, though. How can Ivar find his true love when he is absolutely convinced she died almost a thousand years ago?
Makenna is a former combat medic who survived the horrors of war, only to come home and realize she no longer fit in there. She was too changed. Searching for a way forward, while running from the nightmares of her lost brethren, she finds what she’s looking for in the depths of the serene Sapphire Lake. Makenna isn’t one to shy away from a fight, but will she fight for the love of the merman who saved her life and captured her heart?