This image is from Grotte Cosquer. I think that it is the symbol for water, with a horizontal line added on the left, which may mean water level.
A molecule of water looks something like this diagram. It is a bent molecule with a Λ shape. However, this shape could not be used as the symbol for water because in Cave Script, Λ is used as a pointer to indicate the sequence of events.
So what is the symbol for water?
It is based on the molecular structure of ice (solid water). The image on the left depicts an ice crystal layer. It is made up of hexagons, but your eyes should focus on a zigzag line. Hexagonal ice (ice Ih) is the structure adopted when ice forms naturally on Earth.
The symbol for water survived in many cultures for a long time. This is an example on a cauldron from Cishan in China (on display at the National Museum of Chinese History, Beijing in July 2012). Its estimated age is 7,500 years. The double symbol water, water seems to be justified by the container’s large size.
Engraved water symbol: Jean Clottes, Jean Courtin, Luc Vanrell: Prehistoric Images and Medicines Under the Sea: http://www.bradshawfoundation.com/cosquer/cosquer2.php
Water molecule and ice crystal layer: Durham University Physics Department: National Science and Engineering Week 2010: The structure and properties of ice: http://massey.dur.ac.uk/NSEW/properties_of_ice.html
Water vessel: Photo: Lynn Fawcett: Display at the National Museum of Chinese History, Beijing in July 2012.