What is "physicist's water"?
I suspect the author might refer to &quot;physicist's water molecule&quot; term in order to underline the fact the geometry and energy of this molecule has been introduced as is, and hasn't been affected by intra- or intermolecular interactions.
I believe there is an allusion to Ancient Greece's philosophical schools which treated natural phenomena such as air, fire, water etc. as the purest and simplest entities. The term &quot;physicist&quot; itself refers to pre-socratic philosophers (Ionians, mostly) which abandoned religious attempts to formalize the reality [1, p. 20]:

... Pre-Socratic philosophers [...] have to be called Physicists because they abandoned the civil religion, whose insistency deified the city’s eponym, the royal function or the legislator’s role, the valiant warrior or the fertile woman, because they disdained what the social projects into the religious, whereas they kept or invented a global religious, that divine that’s immanent to the universe.

For instance, based on what's been considered the most fundamental element, physicist’s water has originally been introduced by Thales of Miletus, just as physicist’s fire &quot;belongs&quot; to Heraclitus.
Reference

Serres, M.; Burks, R. Geometry: the third book of foundations; Bloomsbury Academic: London; New York, 2017,. ISBN 978-1-4742-8139-3. (Google Book)