Polarized lenses absorb horizontal light. Because the light source is coming from above and typically reflecting from a flat surface below, the majority of glare is horizontal light.
By removing that incoming light with a polarized lens, you’re left with only vertical light. The result is the ability to look through the surface of the water with far less reflecting light interfering with your view.
Think of it as being able to utilize the ambient light under the water without a light shining down on it and flashing back into your eyes
The increased visibility makes sight fishing possible, even in bright conditions. It allows you to see underwater landscape features you may have otherwise not known were there.