"So the things in the water won't do anything to us if we cross in Voldemort's boat?"
-- Harry Potter, hopefully (HBP26)
The underground lake was located in the sea cave where Voldemort hid the locket Horcrux. The lake was clear and still, but beneath its surface floated hundreds of Inferi. When the water was touched by a human hand, the Inferi would rise and attack. In the center of the lake was an island with a stone basin on a pedestal. The basin held potion which protected the Horcrux. The only way across the lake was a small boat, only able to carry one wizard. The boat, however, was enchanted to detect the magical power of its occupants, and because Harry was underage his magical power was not taken into account. He and Dumbledore were able to ride across the lake together.