As of Minecraft Java 1.9 Snapshot 15w34a, throwing Water Splash Bottles have been able to extinguish fire blocks. As described on the Minecraft Wiki:
"Splash water bottles will have no effect on almost all entities, but will put out fire in the block hit and the four blocks horizontally surrounding it."

As of 1.14 Snapshot 19w08a, Splash Water Bottles no longer extinguish fire blocks. They do still however extinguish campfires and deal damage to Blazes and Endermen as intended.

To test:
1. Light a fire.
2. Throw a Splash Water Bottle onto the fire.