Well, being pedantic, my favorite hobby:

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')