Chances are that deaths would have been fairly uncommon, unless something truly catastrophic happened. Ships in the Viking Age were fast, and they could cross the North Sea in two or three days in favorable weather. In unfavorable weather, they would probably just stay ashore, seek the nearest possible shelter, or if necessary, travel with the storm. This minimized their risk of a catastrophic shipwreck, and time at sea was generally too short for disease or malnutrition to take a toll.

Viking ships seem to have been built in such a way that there was little risk of being swept overboard. They used round stones as ballast, which settled with the pitch of the boat and gave it stability even in rolling waves. Moreover, bulwarks were relatively high, and the rigging overhead forced crews to kept their center of balance low. Even in bad conditions, there was small chance of being lost at sea.

Perhaps the biggest danger was accidental injury. The sail was positioned directly amidships, and sudden changes in the wind could rake rigging and sail through the crew areas. This was especially dangerous when weather conditions strained the rigging. Viking ships were built almost entirely of organic materials -- such as wood, rope, wool, and leather -- and many of these could fail suddenly and with explosive force. Injuries would be rare, but they could be deadly.

Archaeologists have done a superb job reconstructing what it was like to sail on a viking ship. In 2007, they sailed a replica warship from Denmark to Dublin for display. BBC did a terrific documentary on the voyage. You can also learn even more from the museum website. And there's another great website (with pictures!) run by Texas A&M.