"It is said that a foul beast lurks in this lake, guarding the entrance to the Mines of Moria. In light of this, does the lake not seem slightly... beast-free to you? I'd like to believe the creature has simply been slain, but I fear in my heart it has simply... MOVED...
I would be far more at ease if its defeat could be confirmed. Perhaps you could investigate other bodies of water?" Boromir upon approaching him.
"You needn't explain... I can tell just by looking at you that you have waged a watery battle. That, and I just saw the beast resurface in the lake a moment ago. Excellent work! Let us hope it was the SAME BEAST, lest this whole ordeal have been in vain! Hahaha! ... Ahem. Oh yes! Your reward." Boromir upon completing the quest.
Investigate the underneath of Mines of Moria and defeat the beast.