Lars Lake is a body of presumably fresh water, located on the Isle of Berk. It appears to have some depth to it, based on Hiccup's Diving Bell testing there. The lake appears to be surrounded by rocky cliffs, but does have at least one small sandy beach area.
In the episode "Tunnel Vision" screen the Whispering Death drains Berk's well, Hiccup instructs the Twins to go to Lars Lake to fill as many canteens as they can in order to bring fresh water back to the village. Tuffnut jokes about how big it is, comparing its size to Ruffnut's butt.
In the episode, "Dire Straits", Hiccup and the Dragon Riders go to Berk because of a famine affecting the people. No traders are coming through the Straits of Baldur because the Dragon Hunters chained a Submaripper there. In order to stay under water to try and free the Submaripper, Hiccup and Fishlegs concoct a Diving Bell device in Gobber's blacksmith shop with a small window port of Death Song Amber and test it at Lars Lake. The first test showed that one layer of amber was not enough to withstand the pressure of the water.