So the cause of scurvy isn't sailing a lot per se, it's not having access food containing vitamin C for long periods of time (symptoms start after about a month).

The Norse were sailing around the Baltic Sea, the North Sea, over to Iceland and Greenland, and along the coast of Europe for the most part. Many of these journeys would not take them very far from land for that long. So it was probably less of an issue than for sailors from the late fifteenth century on, who were spending much more uninterrupted time at sea.

However, they did sometimes make long voyages and there were times on land when fresh food was scarce, long winters and such. They were aware of scurvy, which they called "skyrbjugr." I believe the first recorded Norse mention of it is in a saga from 1000 AD about Thorstein the White, when a man demands compensation for being mocked for having scurvy at sea (see pg 72 of that article). They seemed to have been aware that it could be treated by certain vegetables and berries.

As for navigating at sea, they had various approaches. If possible, they might stay close to land where they could navigate by visible landmarks. Dead reckoning was another option, where they used estimates of what direction they were going, at what speed, and for how long to determine where they were (as you might imagine, this is not very exact and is particularly difficult over long distances or under stormy and foggy conditions). Losing your reckoning, sometimes for days, was a common problem in the sagas. Additionally, they could use natural phenomena like the positions of the sun, moon, and stars, the presence of sea birds that live near land, knowledge of local currents and weather patterns, etc.

There's some speculation that they may have used sunstones, crystals with polarizing properties, as a navigational aid to find the position of the sun on a cloudy day. Something like this is mentioned in an Icelandic story, Rauðúlfs þáttr

"He sent a man out to observe the weather, and there was not a patch of clear sky to be seen. The King then asked Sigurð to determine how far the sun had traveled. He gave a precise answer. So the King had the sun-stone held aloft, and observed where it cast out a beam; the altitude it showed was exactly as Sigurð had said."

But it's unclear whether these were ever actually used for navigation. It's possible, we just don't have any direct evidence for it that I know of.