Prob not arson, but intent matters. If you know pouring water into a burning ev will make the fire worse then no doubt there will be something you can be hit with.

It may be suicide.

Arson in the second degree is defined (in my state) as starting a fire or causing an explosion and it causes damages of more than $750.

So yes, if the water causes a fire or explosion, it can meet the definition of arson.