I'm just sitting here waiting for someone to mention 'Daughter of the Pirate King'. Because, wow, was that a letdown on the slow burn front. It gets recommended a lot for pirate romance, but the enemies-to-lovers switch flips way too fast for my taste. The adventure is fun—she's deliberately captured to search her captor's ship—but the romantic tension deflates quickly.
If you want a slow burn that actually smolders, I'd point you toward 'The Salt Roads' by Nalo Hopkinson. It's not a straightforward romance; it's a complex, multi-perspective historical fantasy following the lives of enslaved and free Black women across centuries. The pirate element comes through in the section about Jeanne Duval, and the relationships, including a romantic one, are deeply woven into a struggle for freedom and identity. The 'burn' is in the emotional resonance, not just will-they-won't-they.