Sexual symbols in the movie
Sexuality and especially distorted sexuality is one of Polanski's favorite theme throughout his entire career.
In "Knife in the water" these two elements/objects are sexual in themselves. Knife is a fallic symbol, water is feminine symbolizing wetness.
Two males playing a sexual game with the knife putting it among the spread fingers first slowly than faster and faster is a symbol of the penetration and intercourse.
Knife falls into the water.
Other elements:
The Boy is trying to harness the mast, symbolizing his raging sexual drive.
I saw this movie ages ago so my memory needs refreshing. Any other scenes that come to your minds?