A ski backpack with no hydration system simply lacks any built-in sleeve, hose routing, or dedicated storage for a water bladder. You'll carry water in a bottle inside the pack or in your jacket pocket, which means stopping and taking off your pack (or reaching awkwardly) to drink.
This setup works fine for short resort days where you're never far from a lodge, or if you prefer a very small, low-profile pack. Many riders just stash a water bottle in their pack for quick chairlift laps and don't mind the inconvenience.
However, for backcountry touring or long days without lodge access, stopping every time you need water gets old fast—and you're more likely to skip drinking, which leads to dehydration in cold, dry mountain air. If you go this route, make a habit of drinking at every transition or rest stop.