Because you can craft mundane splash potions, I gave them a use.

• /scoreboard players set @e[type=ThrownPotion] waterPotion 1 {potionValue:16384s}
• /execute @e[type=ThrownPotion,score_waterPotion_min=1] ~ ~ ~ fill ~1 ~1 ~1 ~-1 ~-1 ~-1 minecraft:air 0 replace minecraft:fire
• /execute @e[type=ThrownPotion,score_waterPotion_min=1] ~ ~ ~ fill ~1 ~1 ~1 ~-1 ~-1 ~-1 minecraft:cobblestone 0 replace minecraft:lava 4
• /execute @e[type=ThrownPotion,score_waterPotion_min=1] ~ ~ ~ fill ~1 ~1 ~1 ~-1 ~-1 ~-1 minecraft:stone 0 replace minecraft:lava 2

They put out fires, and turn flowing lava to cobblestone and smooth stone (do not affect lava source blocks)