Water-resistant knee pads are the best all-around choice for most skiers. They feature a DWR (durable water repellent) coating or water-resistant outer fabric that sheds light moisture and snow, keeping the padding from soaking up melt and getting heavy and cold against your legs.
For typical resort days—groomers, light powder, and mixed conditions—water-resistant pads strike the right balance. They keep you comfortable and dry without sacrificing breathability the way fully waterproof shells sometimes can.
The main trade-off is that in deep powder or heavy wet snow, moisture can eventually work through. If you're a powder hound or ski mostly in spring slush, waterproof pads might be worth it. But for most skiers, water-resistant is the sweet spot.