Water on Li-Ion battery fire: good idea, bad idea, or neutral?
Oregon State University published this Lithium Fire Prevention Fact Sheet.

For Li-Ion batteries:


  If formally trained, you may use a st
  andard ABC fire extinguisher or water to put out a lithium ion battery fire.


For batteries containing elemental Lithium:


  
  Only Class D fire extinguishers that contain a copper powder are approved for combating a lithium fire.
  DO NOT USE WATER OR ANY OTHER TYPE OF EXTINGUISHER BECAUSE ORGANIC &amp; INORGANIC LITHIUM METAL FIRES REACT HIGHLY WITH WATER AND COMBUSTIBLE SUBSTANCES.
  


The paper also strongly recommends that you "Let the fire department fight fires." In other words, you should probably only fight them yourself if you absolutely need to.

For more information, take a look at the Lithium Battery Safety and Handling Guide.