Final answer:
Hrothgar's coastal sentinels thought Beowulf was a Viking raider, which aligns with the historical context of Vikings and their feared seasonal raids facilitated by their advanced longships.
Step-by-step explanation:
Hrothgar's coastal sentinels initially think that Beowulf is a Viking raider. This is understandable considering their experience with Norsemen, who were often referred to as Vikings and were notorious for their seasonal raids along European coasts and rivers. The Viking longships, such as the Oseberg ship, were fast, seaworthy, and capable of both oceanic and river travel, making them a formidable tool for Viking expansion.
Thus, the sentinels' first assumption about a group of men arriving by ship would naturally be that they were facing a potential threat from these well-known Scandinavian warriors.