A lake may symbolize the unconscious, particularly if it is set in the deep, dark hollow of high mountains.
It may be a symbol of self. The stillness (and clearness) may suggest the deep self that lies beneath the turbulent emotional self.
Lakes symbolize the Earth’s eye, through which the inhabitants of the Underworld are able to gaze upon humans, animals, plants and so on. Marshes symbolize those eyes which have wept too much.
In Egypt a vast lake filled the Wadi Fayum. ‘Local theologians in the Late Period regarded it as the manifestation on earth of the Cow of Heaven, a liquid sky in which... the Sun... had mysteriously hidden... the overflow from the Primordial Ocean... “Mother of all the gods, giving life to men”.’ Artificial lakes were dug beside temples. Their banks were the scene of nocturnal mysteries and in their waters the priests performed their ritual ablutions. They symbolized the abiding powers of creation.
The Gauls regarded lakes either as deities or as the abode of gods. Into their waters they cast offerings of gold and silver, as well as trophies of their victories.