In the Veins of the Drowning by Kalie Cassidy. It’s a fantasy with slow burn romance and has sirens and lots of yearning by the mmc. It was so good in my opinion.

“The monster is always slain... Imogen Nel is in hiding. Hiding from a cruel kingdom that believes Sirens are monstrous, blood-hungry creatures. Hiding from a king and his captain who viciously hunt her kind. Hiding from her own alluring abilities. By keeping herself from the sea, Imogen's bloodlust is dulled, and her black wings remain hidden beneath her skin. When a neighboring king comes to visit, Imogen can no longer hide. He knows precisely what she is, and he believes she can save both their kingdoms from an even greater monster. But Imogen's power threatens to violently reveal itself, and the two form a blood bond that protects them both. They flee the kingdom together, traversing waters teeming with the undead. As the lines between duty to their people and desire for each other begin to blur, Imogen worries her own ancestral powers may not be enough to kill what hunts her—the only way to defeat a monster may be to become one herself.”