Full Idea
If they do count as water, individual H2O molecules are the smallest items that can qualify as water on their own account. Hydroxyl ions and protons, in contrast, qualify as water only as part of a larger body.
Gist of Idea
Maybe water is the smallest part of it that still counts as water (which is H2O molecules)
Source
Robin F. Hendry (Chemistry [2008], 'Micro')
Book Reference
'Routledge Companion to Philosophy of Science', ed/tr. Psillos,S/Curd,M [Routledge 2010], p.524
A Reaction
As Aristotle might say, this is the homoeomerous aspect of water. This is Hendry's own proposal, and seems rather good.
Related Idea
Idea 17484 Maybe the nature of water is macroscopic, and not in the microstructure [Hendry]