Answer
Light reflecting from the water is partially horizontally polarized.
Work Step by Step
As explained on page 1096, light reflecting from the water is partially horizontally polarized. Sunglasses have a vertical transmission axis, so in their normal orientation they block the glare.
However, if you lie on your side while wearing the sunglasses, the pass axis is now oriented horizontally, and the glasses will admit the reflected light without attenuating it.