The Patient in Cabin C — Mignon G. Eberhart

The trip aboard the FELICE began as a pleasure cruise. Attractive young Sue Gates was certain that the yacht's owner Monty Montgomery intended to announce their engagement.
Then Terror seized the ship. Monty almost lost his life when he was pushed overboard, the yacht's steward was mysteriously killed, and the ship's engines stopped dead.

The Passenger list included both lovers and enemies: icily beautiful Celia Hadley...naval officer Stan Brooke...bedridden millionaire Sam Wiley...and Monty's sultry half-sister Lalice. But their intrigues soon gave way to a nightmare of death---for now Sue, Monty, and the others were at the mercy of a murderer as cold as the shrouded sea they sailed...