Yup lots of water is very handy - even a bucket full. I once put out a burning motorbike on the side of the road with 6x 1.5L of Buxton water I had in the back of the car - owner was very gratefulIt's hard to beat a hose pipe tbh although co2 would be the type of extinguisher I'd go for if i could afford one. I just pull my garden hose in when I'm welding. Compressed air never usually fails to put small fires out as it typically cools the materials rapidly but you have an endless supply of water with a hose which will be fine for most car related fires. Fire extinguishers can be exhausted fairly fast as they're primarily for executing an escape rather than putting the fire out completely. As such I'd say always have the hose even as a backup for all eventualities.
