Every school year at Hogwarts, first years sail across the lake to the castle led by Rubeus Hagrid in boats as per Hogwarts tradition. When they got to the school, they are then Sorted into their houses.
Students from Durmstrang Institute arrived at Hogwarts on their enchanted boat via the Hogwarts 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 hold his breath for that long, while Cedric and Fleur used the Bubble-Head Charm, and Krum partially Transfigured himself into a Shark.