How is Beowulf's story about the swimming match with Breca different than Unferth's version of the tale?
Of all of Hrothgar's men, Unferth is the most mysterious. While everyone else rejoices at Beowulf's arrival, Unferth tries to discredit him. Jealousy is a terrible thing, and Unferth's jealousy causes him to twist an adventure from Beowulf's youth in order to make Beowulf look like a coward.
Unferth first calls Beowulf a fool who engaged in a swimming match with Brecca against the advice of the elders. According to Unferth's version, Brecca won the swim-off and went on to be a beloved ruler in Southern Norway.
Beowulf doesn't call Unferth out for being a liar, but rather blames the alcohol before proceeding to tell the story of what actually happened: Beowulf and Brecca did go swimming against the advice of the elders, but after five days of swimming, they encountered sea monsters. Beowulf slew the monsters (nine in all) and swam to safety on the Finnish coast.
Welthow, Hrothgar's queen, saw Beowulf's true story for what it was, and toasted his health and success in destroying Grendel.