Is it arson to pour water on a Tesla if it catches on fire?

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.

Lot of misinformation here. Water is your best bet for lithium ion battery thermal runaway. There's a small risk of shorting out other cells, but that doesnt matter once its on fire anyway. Lithium ion batteries contain lithium bonded to a metal oxide. As the name implies, the ions travel between electrodes, but the lithium is bonded/stable on either side. Lithium metal batteries are the ones that blow up in water. However, you typically only see those in small coin cells (watch batteries) since they're not (currently) rechargeable.

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.