Many historians consider the Viking longship to be the finest invention of the Dark Ages, which isn't saying a lot. But they were sea worthy in that the hulls flexed nicely in heavy seas, and with oar power and a shallow draft
they could go almost anywhere. Sometimes they would sail them up rivers. Once a large fleet of longships showed up in Paris and the Danes wouldn't leave until they got paid in gold, which the Parisians eventually did. But those longships also sank in good numbers. Many of the Viking sagas had comments about how "Twenty three ships sailed for Greenland
, and 12 made it." There is a Viking ship museum in Roskilde, Denmark
. They build replicas and sail them. Great place to visit if you ever have the time.