Some people suggested using hotspots alteration like the " a hotspot takes 2 action to be extinguished". (I have to try that one) but I've been thinking on making the fire spread more intensely. As a bonus this alteration would indirectly nerf the Water Car. So here it goes:
When you hit a hot spot with the fire spread (rolled dice at the end of the turn) resolve that spread as normal then:
1 - Spawn a smoke/fire/explosion on the opposite side (as if you turned the dice upside down)
2 - Roll the dice again and place another hotspot (not fire or anything).
# - When an explosion occurs, put a hotspot in it's center.
This basically makes harder for the water truck to control an out of control fire because it will spread to the other side of the building as well.