In the 01960s and 01970s it seemed like it might sometimes polymerize, which might have implied that it's always somewhat polymerized:
https://en.wikipedia.org/wiki/Polywater but after 12 years of research this turned out to be wrong. If someone got similar experimental results today, while taking more care to prevent the contamination that gave rise to the original results, and did it under different conditions that hadn't been intensively investigated during the original polywater period, it would definitely merit further investigation.
In 01988 Jacques Benveniste did some experiments which seemed to show some sort of chemical alteration of water in the presence of antibodies, though it didn't show up with the usual spectroscopic techniques: https://en.wikipedia.org/wiki/Water_memory which were published in Nature. Replication attempts were made by several teams. They failed, so Benveniste was wrong.
So I wouldn't say it's "settled science" that water is H2O. When contrary evidence pops up, chemists investigate it and find out what's wrong with it; they don't just dismiss it. That would be unscientific.
We could also quibble about the meaning of "H2O", though perhaps this is less interesting. For example, although it's only about 0.1 ppm, a part of any sample of water exists in the form of free hydronium (H3O+) and hydroxyl (OH-) ions, not H2O. Because this is an equilibrium, you can't just remove the ions to get pure H2O; they come right back. This self-ionization is responsible for significant aspects of water's behavior, including the formation of streamers that limit the storage time of water-dielectric capacitors, and the particular way acid-base chemistry works in water.
Natural water also contains about 155 ppm DOH and D2O, though this can be removed, and it's arguable that this is within the meaning of "H2O" anyway.
And the water molecules in liquid water or ice form a continuous hydrogen-bonded network, though this was not known until 01920; to some extent it's just a matter of convention rather than objective fact that we consider those bonds to be "intermolecular", making water H2O rather than an (H2O)n polymer; we consider the weaker hydrogen bonds between DNA strands to make DNA a single molecule rather than two intertwined molecules. (The polywater affair, however, posited the formation of a covalent water polymer.)