As far as I'm aware, there is no separate word for freezing water -- i.e. water that is very cold and will, if it continues to get colder (and has something to crystallise around), turn into ice.
So the symmetry seems complete: ice -> freezing water -> water -> boiling water -> steam.
Basically as it gets colder water exchanges energy with the environment and gets colder.
But once it reaches freezing temperature, it can no longer get colder and all the energy is used for the formation of crystals.
> But once it reaches freezing temperature, it can no longer get colder and all the energy is used for the formation of crystals
Water at freezing temperature can get much colder without freezing. https://en.wikipedia.org/wiki/Supercooling:
“Water normally freezes at 273.15 K (0.0 °C; 32 °F), but it can be "supercooled" at standard pressure down to its crystal homogeneous nucleation at almost 224.8 K (−48.3 °C; −55.0 °F).”
But the semantic point still stands. Boiling water is still water -- in the specific sense of H2O in its liquid state -- while ice is not. The complaint that frozen water has a single-word synonym while boiling water does not is making a false equivalence.
Frozen water represents a state change and that different state commonly gets its own word: ice/water/steam equates to solid/liquid/gas
Boiling/freezing water represents the state of the liquid, not the transition. Its descriptive. Water boils away into steam, or freezes into ice.
Should we consider luke-warm water also singular? What about body-temperature water? cool water? It makes sense not to treat adjectives/descriptive words combined with the subject as singular because the definition already exists in the root of the words (meaning of adjective word + meaning of subject word). Blue clay is another example, why would that be a singular?
It really only makes sense to me in the rare cases where the combination words represent something different or non obvious than the combined meanings of the two words (i.e to 'give up')
We have a lot of words for "frozen water" because it takes a lot of forms. As far as I know "boiling water" is only one thing so we've never needed additional words to distinguish it.