Communing with Mr. Quin: The Voice in the Dark by Agatha Christie

This Harley Quin story has, if not quite everything, then nearly
so: a boating catastrophe in the distant past that is neither the
Titanic nor the Lusitania; a young Mr. Satterthwaite (!!) snogging
(!!!!) a maid (!!!!!!); a man named Bimbo (seriously); and a true
rarity for Christie—a bathtub murder. Things always get a little
weird in this corner of the Christieverse (the Satterthwerse?), so
fasten your seatbelts... (oh yeah--Bette Davis gets a shoutout
too).