This is the 1945 film Leave Her to Heaven (20th Century Fox), a murder-suspense picture directed by John M. Stahl, and starring Gene Tierney, Cornel Wilde, Jeanne Crain, Vincent Price, and Darryl Hickman.
In the film, Tierney is a mysterious woman who marries writer Wilde. Wilde's crippled younger brother, Hickman, lives with him in their lovely log cabin mansion in the woods. Tienry proves psychopathic, and slowly kills everyone in Wilde's life who has a claim on his love.
Here she is rowing that pretty little Darryl Hickman out onto the lake.
Here's Tierney helping cute little Darryl put on some sunscreen.
LOOK OUT, DARRYL!!!!!!!!!!!!!!!!! She's gonna stick her CLAWS into your back!
In this scene, she's contemplating whacking him over the head with an oar and leaving him to the fishes!
Okay, so what she really does is over-tire him and let him drown. But she was CONTEMPLATING hitting him. She's evil.
You know she's evil, because she wears sunglasses while young men drown.