Polarized lenses work by filtering polarized light. Sunlight, by itself, is not polarized. But as it bounces off a flat, reflective surface, it can become polarized. So polarized lenses can help to reduce the glare that you get on the surface of the water. And this can help you see deeper into the water.
HOWEVER, it really only works if the sun is behind or in front of you. If the sun is off to the side, they won't really work (well sometimes if you tilt your head at the right angle they can). And if the water is stained or muddy, it won't help you to see through it any better, as polarized lenses do nothing to help that. So the sun's got to be at the right angle and you've got to have somewhat clear water for it to work better than regular sunglasses.
So it's not just a marketing gimmick. It actually works. To see a better example, look at an LCD screen through polarized lenses. The light coming off an LCD screen is polarized. So usually, if you look straight on, you can see clearly through the lens. But if you spin the lens around, the screen will go dark and then lighten back up depending on the angle. Usually, they're set up so that the screen darkens when the lens is turned 90°. But sometimes, like on my fish finder, it's less. My fish finder darkens at about 50° clockwise, so I sometimes have to tilt my head to the left to see it better with my polarized lenses on. That's a design flaw Lowrance should have anticipated and corrected, but whatever.
Polarized lenses don't block UV light by their nature. They need a special coating to block UV light. So while some polarized lenses will block UV light, not all will. And since it's just a coating (or sometimes the material of the lens itself), it's not uncommon to find non polarized lenses that also block UV light. They even make regular glasses that aren't sunglasses that block UV light. You have to look for that feature separately and not just assume that they always go together.