as discordant as fire and water; complete opposites
Idiomatic noun describing two things or people that are fundamentally incompatible, like fire and water. Often used in the phrase 火水の仲 (ひみずのなか) to describe a relationship of extreme discord.
Those two are like fire and water—they just don't get along.
彼の意見と私の意見は火水のように相容れない。
His opinion and mine are as incompatible as fire and water.
Compound of 火 (ひ, fire) and 水 (みず, water), reflecting the traditional view that these two elements are opposites and cannot coexist harmoniously.