Dried fish (harðfiskr), bread, meat preserved in whey, and whey itself would have been commonly eaten aboard ships, if there were no islands near enough to stop at. That said, though, in most of the sagas where there is a long duration of shipboard travel, we don't get any descriptions of the food that is eaten.

To follow up on this, how long was the typical Norse voyage? How long would Viking raiders be away, and was that different from trading voyages?

Could they have caught fresh fish?

Thanks for the responses, everybody <3.

I want to become a viking now.