Execute command in entity with potion effect
Solution 1:
You will need to assign a label based on the mob's NBT data using /scoreboard first, and then target based on that label:
/scoreboard players tag @e[type=Creeper,tag=wb] remove wb
/scoreboard players tag @e[type=Creeper] add wb {ActiveEffects:[{Id:13b}]}
/effect @e[type=Creeper,tag=wb] minecraft:instant_damage 1 1
Solution 2:
This can be done in just one command in Minecraft 1.13
/effect give @e[type=creeper,nbt={ActiveEffects:[{Id:"minecraft:water_breathing"}]}] instant_damage 1 1`