The task did indeed ask each champion to retrieve something from the Black Lake. Each champion had someone important to them suspended below the water, and it was their responsibility to rescue them. Apparently, the judges of the event thought this might be traumatizing to the families of the champions, so they were not allowed to attend. The blonde from Beauxbatons was given her young sister to save, Cedric Diggory was given Cho Chang, and Harry was to save Ron. He didn't need Arthur to tell him who Victor Krum was tasked to save. 