The first two thirds is a shockingly benign and plain thriller with minimal tension or complexity despite the central horrific acts driving the story.
But then the last half hour kicks in. Dark, violent, scary, tense, disturbed. The use of shadows to frame and imply the violence adds pounds of atmosphere and dread to the final confrontation, as everything occurs in semi-dark. It's masterful cinema but the preceding scenes feel like they are of a different film.
Mitchum's performance really shines in this last 30 too, as he performs primarily through his physicality, cutting through the water like a crocodile, looming over his victims, in dogged pursuit of them.
I spent so much time watching this, wondering why it's so…