Beowulf swimming underwater for hours while he searches for Grendel's mother's cave is a perfect example of a superhuman trait of Beowulf.
Supernatural Forces are included in the story
Battling Grendel's Mother Underwater shows great courage, valor, and motivation on Beowulf's part.
Writing style is ceremonial
Grendel's Swamp, where he dragged his victims back too.
Main characters give long, formal speeches
The Dragon that is killed by but also takes the life of Beowulf, is another example of Supernatural forces in the book.
The writing style, and therefor the actions of the characters is very ceremonial during Beowulf's funeral. As we see in typical Midevil funerals from movies and such, the victim is engulfed in flames. |n the book they state that they believe his sole will be carried to the heavens in the smoke.
Beowulf introduces himself to Hrothgar with a long, formal speech. Also known as his brag to show Hrothgar that he is the perfect man for the job. Which is to slay the evil Grendel which has been haunting his kingdom for so many years now.
"Behold King Hrothgar, It is I Beowulf. I come in peace and seek the death of Grendel."