The Rhine is quite navigable. One of the amazing things about the Vikings is how good they were at building ships. As I understand it, the same ship could be used on rivers, lakes or crossing the ocean to Iceland, Greenland or Vinland (Newfoundland).

A viking ship laden with trade goods could sail down the Humber from York in northern England, cross the North Sea, up the Neva River into Lake Ladoga, down into the lakes and streams drained by the Dneiper and then into the Black Sea to Constantinople.

There wasn't much that could be done for coastal defense. The Franks had no navy and they probably didn't have the industrial base for booms at that point.

From what I understand the response to the Vikings was the organization in England, of the shires (now counties) and hundreds -- essentially all the men in the kingdom became part of a militia that could be raised quickly. They also built beacons on hills that could be set alight quickly to summon help. On the continent, the classic form of feudalism was developed with much the same idea: the quick raising of an armed force.

Of course, these don't seem to have been that effective: a Viking under Rollo forced the King of France to cede Normandy to him and Vikings extensively settled parts of England (the Danelaw). England was briefly incorporated into a Danish Empire under Canute.

The real winning strategy was Christianization combined with intermarriage, so Viking identities were replaced with English and French ones. It only took a few generations, too. William the Conquerer became King of England about 150 years after Rollo died and while he was Norman he wasn't considered to be a Viking.