I had the same question after finishing the book! The cover and marketing gave it that 'ripped from the headlines' feel, but Ruth Ware confirmed in interviews it's purely fiction. She's said the spark was a hypothetical thought about being trapped on a small boat with a potential murderer, not a real case. The tabloid-style details and the social media frenzy depicted felt so real it had me searching for news reports for a solid hour.
That authenticity, I think, comes from her research into luxury yachts and isolated cruise ships. The details about cabin layouts, the claustrophobic atmosphere, the crew hierarchy—all that rings true because it's well-researched, not because it happened. It's a classic locked-room mystery premise, just updated with modern anxieties around influencer culture and being constantly observed yet utterly alone. I was almost disappointed to learn it wasn't real; part of me wanted that extra layer of creepy truth.