The Fisherman at Falcon Point is a short story first published in the Arkham House volume The Shuttered Room And Other Pieces (1959). One of the "Posthumous Collaborations" by August Derleth. Based on Lovecraft's Commonplace Book entry: “Fisherman casts his net into the sea by moonlight—what he finds.”
Everything else in the composition was under Derleth's control. It may also be influenced by traditional mermaid/selkie legends.