From what I know, there aren't many harem novels with the theme of One Piece. However, I have a very good recommendation for you. It's One Piece: The Strongest Surgery Assistant. The author is Withered and Crimson. Although the harem elements were not the main thing, the emotional lines were very good, and the protagonist's experience as a surgeon was also very exciting. It was worth reading. I hope you like my recommendation, Mwah ~π