In spite of the title this always put me to sleep
If I remember well (I saw the movie at a theatre in the early '70's and again on tv in the '80's. I've got the dvd but still to watch) there's a murder and a mystery about what happened to the corpse. But I may be wrong.
There is no murder. You may be thinking of one of the other sailing films. Plein Soleil, perhaps?
A bit too much time is spent on our protagonist's romantic foibles for my taste though it serves a purpose.
The boy does pretend to drown at one point, but there is no actual killing. And certain tensions are present among the characters (which intensify and then slacken, and then intensify again) but matters never rise to the level of murder or even attempted murder. To call this a thriller would be the same as calling Crime and Punishment a policier.
Daredevil (2003) I saw that this was particularly criticized. Can't see why: I think that is just as good (or bad) as the other Marvel based movies. 6\10