The Great Lake

Every school year at Hogwarts, after the Hogwarts Express arrives at Hogsmeade Station, Rubeus Hagrid (or a substitute) calls the first years to follow him to the lake shore and the magic boats. Whereupon Hagrid occupies one boat and each other boat carries up to four students. The boats then self-propels across the lake to a dock under Hogwarts Castle as per tradition. (at the start of the 1994–1995 school year, first year Dennis Creevey said he fell into the lake and the Giant Squid pushed him back into a boat). Upon reaching the castle, Hagrid and the first years disembark and climb the stairs to the lawn, entering the castle through the front doors. First years then waited until summoned into the Great Hall to get Sorted into their houses. At the end of their first year, students take the boats back to Hogsmeade Station, while most older students ride down the lane in carriages.

Students from Durmstrang Institute arrived at Hogwarts on their enchanted boat via the Great Lake. As part of the Second Task of the Triwizard Tournament, the four champions Harry Potter, Fleur Delacour, and Cedric Diggory, and Viktor Krum had to swim in the lake for one hour to rescue something that was taken from them. Harry used Gillyweed to be able to breath underwater, while Cedric and Fleur used the Bubble-Head Charm, and Krum partially Transfigured himself into a shark.

Dumbledore's Funeral
In 1997, Albus Dumbledore was laid to rest in a white tomb on the shores of the lake following his death in the Battle of the Astronomy Tower.

Trevor's escape
At some point in his life, Trevor managed to escape from Neville and into the lake. Both pet and owner felt a sense of relief at this.

Inhabitants
The grounds of Hogwarts function partly as a nature reserve for magical creatures which have difficulty existing in Muggle-inhabited areas. There are Grindylows (vicious little water demons), Selkies (of a hardy Scottish strain) and a giant squid, which is semi-domesticated and permits students to tickle its tentacles on sunny days, when it basks in the shallows.

Temperature
The lake is often said to be very cold, but Viktor Krum was known to have swam in it, wearing nothing but swimming trunks, apparently not affected by the low temperature (although this was probably due to the fact that his school, Durmstrang, is located in a place that is even colder).

Lily Evans and her friends were also said to dangle their legs over the water, which could mean that there were no rules whilst they were in school against swimming in the lake.