"Myrtle . . . what lives in the lake, apart from the giant squid?"
-- Harry Potter (GF25)
A large, freshwater lake (a loch, actually, since this is Scotland) lies to the south of Hogwarts castle, which stands on a cliff overlooking the water. The lake has a number of very unusual inhabitants, including a giant squid, a village of merfolk, and many grindylows. The lake is approximately a half-mile in diameter (GF26). The lake is also known as the Black Lake.
On one side of the lake is Bowtruckle Island where there are a few trees, one of which contains bowtruckles (CG70).
Harry and other first years arrive to Hogwarts Castle by boat in the lake.
In Goblet of Fire, a ship carrying students from Durmstrang rose from the depths of the lake and glided to the shore.
The lake is the location of the Second Task of the Triwizard Tournament.
Dumbledore's burial takes place on the shore of the lake.
From the Web
On Pottermore, J.K. Rowing adds her own thoughts about The Great Lake.