I thought maybe oxygen. Water is H2O, but oxygen also appears as an essential part of fire. And fire dies with water. But it’s not a great answer because the oxygen part of fire specifically doesn’t die with the death of fire, it actually becomes more preserved due to the reactions no longer transforming it.
Apparently ice is the answer. Even that doesn’t hold up to scrutiny though because if you put ice into an ice and water mix that’s going through a phase change between solid and liquid, it’s not necessarily going to die unless heat is added to the point that the mix completes phase change as liquid water. But it could theoretically go the other way and grow if the heat is removed, and the mix could completely become ice. But that’s a bit nitpicky, in most situations when ice is added to water it does indeed melt