What Happened Underwater

What exactly happened in the second task of the Tri-Wizard Tournament? Under the waters of Black Lake,dwell the merfolk, grindylows and giant squid. And now, the champions of the Tri-wizard Tournament and their loved ones... "You will be awake and yet asleep, with no memory of the what happens underwater..." "No." "Pardon me, Miss Granger?" "I want to remember..." "Do you?"