One perfect couple : a novel / Ruth Ware.

Lyla is in a bit of a rut. Her post-doctorate research has fizzled out, she's pretty sure her contract will not be extended, and things with her boyfriend, Nico, an aspiring actor, aren't going great. When the opportunity arises for Nico to join the cast of a new reality TV show, The Perfect Couple, she decides to try out with him. This is the big break she needs! A whirlwind preproduction process later, Lyla finds herself whisked off to a tropical paradise with Nico, boating through the Indian Ocean towards Ever After Island, where the two of them will compete against four other couples--Bayer and Angel, Dan and Santana, Joe and Romi, and Connor and Zana--in order to win a cash prize. Alas, not long after they arrive on the deserted island, things start going wrong. After the first challenge leaves everyone rattled and angry, an overnight storm takes matters from bad to worse. Cut off from the mainland by miles of ocean, deprived of their phones, and unable to reach the boat and crew that brought them there, the group must band together for survival. Tensions run high and fresh water runs low. Then, one by one, contestants start to die. Lyla suddenly realizes she and Nico are trapped on the island with a murderer on the loose...and nowhere to hide.