The Puzzle of the Silver Persian: Hildegarde Withers, Book 5 (Unabridged‪)‬

Crusty schoolteacher/sleuth Hildegarde Withers can't escape mysterious deaths, even on a trans-Atlantic crossing on an ocean liner. En route to England she spends part of her time (when not feeling queasy) sizing up her fellow passengers. Her intuition helps when passengers and crew start dropping over the side, or just dropping, period.