Dead Calm (1963) by Charles Williams

John and Rae Ingram are alone on their honeymoon yacht in the Pacific, becalmed. It shoud be idyllic. . .but it's not.

On the near horizon a ship is sinking. They rescue its lone passenger, a young man who claims he buried his wife and another couple, dead from food poisoning. But suspicion gnaws at Ingram, a suspicion only too soon justified. Soon Ingram and his wife are nearly overside with the killer's other victims!