Light reflected from surfaces like a flat road or smooth water is generally horizontally polarized. This horizontally polarized light is blocked by the vertically oriented polarizers in the lenses filtering our glare and allowing you to see below the water line.
So polarized sunglasses are a must when fishing but what if you require prescription lenses?
Normally these can be very hard to find and your optician will charge you a small fortune for them. However, there are suppliers online who will provide them with prescription lenses much cheaper than the high street and made to your exact prescription.