Daughters of Triton Tales Series by Nyx Nocturne

Sirenia's the last fertile mermaid of her brood. Racing against a hurricane, she must search out and mate with the only merman who can save her kind—the prophesied Lost Son of Triton. When she rescues injured Navy SEAL Byron Burke from drowning, she seals herself into his life debt. When the time comes to ask him for her freedom, her heart threatens to stand in the way of the last hope for her kind.