In the Old English poem Beowulf, why is Hrothgar's coast watcher concerned about the arrival of Beowulf and his men?
As your question implies, Hrothgar's coast guard is very suspicious when he gets a glimpse of Beowulf's ship:
"Who be ye bearing your weapons,/clad in body-armor, who bring the tall craft/over the sea-street. . . Never more openly have armored men/ begun to land, nor do you know/the word of leave for warriors here. . . ." (ll. 237-246)
The coast-watcher is Hrothgar's first line of defense against an invading army, and in this case, Beowulf and his men not only appear unexpectedly but also fail to give the "recognition" word that would indicate they are friendly visitors. In the absence of the appropriate password, the coast-watcher must assume that Beowulf and his men--who are armed for battle--pose a significant threat to Hrothgar.
In this culture, war is a constant threat, and this threat is realized most often when a ship arrives bearing heavily-armed men without notice. The Viking culture illuminated in Beowulf centers on exploration and war--often, men defend themselves against natural and supernatural beings, but more often, they go to war against each other for territory and riches. The fact that the coast-watcher asks for a "password" indicates that Hrothgar's people are inherently suspicious of any visitors, especially those who are not expected and who arrive dressed for battle.