There are several important scenes involving different bodies of water in Beowulf – the dangerous sea-crossing that Beowulf and his warriors undertake to go from Geatland to Denmark; the swimming contest between Beowulf and Breca and the sea monsters they had to fight; the bloodstained lake, or "mere," where Grendel's mother lives in an underwater cave; and the seaside cliffs where Beowulf slays the dragon – and meets his doom. Why so much water? And why does the water always seem to be associated with, well, really dangerous things?
The easiest answer is that the medieval Scandinavians were a seafaring people. After all, that's part of the reason that the Anglo-Saxons were telling the story of Beowulf centuries later in England – because their Scandinavian and Germanic ancestors had sailed across the sea to colonize Britain.
As a member of a seagoing tribe, Beowulf is familiar with the sea, and also with its dangers. Of course, because Beowulf is an epic, the mundane dangers of the sea – getting swept overboard, getting lost, running out of food and water – are replaced by fantastic dangers, like sea monsters. But the principle is the same. Grendel's mother, in her cave beneath a stagnant lake of bloodstained water, represents the uncertain danger lurking in any watery expedition. Later in the epic, Beowulf's followers will push the carcass of the dragon he slays over the cliff into the water to dispose of it, returning a monster to the place it seems to belong, the dangerous, capricious sea. And the barrow that Beowulf asks Wiglaf to build for him is not just a monument to his memory – it's a monument that can be seen on the coast by men sailing on the sea. In other words, it's a reminder of the strength and success of a hero that you can see and take courage from even in the middle of a dangerous, uncertain world.