This is a loaded thread XD
In the case of trying to decide what constitutes of "knowledge of water", we typically resort to looking at the underlying structure in a scientific manner - H20. However, as has been pointed out, "knowledge of water" in a non-theoretical, but more nominal sense, would dispute that. Additionally, as Quine might point out, if we give up how we understanding the of meaning of water, we lose the separation of the empirical and theoretical assumptions of what water is and thereby any chance at discovering "knowledge of water." In other words, there is no scientific, nominal, or any other way of determining what "knowledge of water" is.
Here are two examples to make this point. Take any definition of water, say, as the "chemical compound of H20" or "whatever fills the water-role" - what if I question the ability of the definer to make the definition? I may be skeptical of the methods used to arrive at the definitions - how can I know that the scientific theories related to chemistry, on which the compounds H20 is based, are valid? You regress into the problem of induction.
This example is not the strongest one. For those of you unswayed, look at this second example:
H20 is also the chemical composition of ice and steam. However we would not
say that is water. So then must we add to the definition, H20 at a certain temperature? This is not the case either, as in differing altitudes, water turns to ice and steam at different temperatures. So we must add barometric pressure to our definition.
However, there are additional conditions I can think of off the top of my head. When I'm driving in the desert, I often see what appears to be water in the distance. Upon further examination I come to find this was not water at all - merely a mirage. So we must add to the definition: ideal conditions for correct perception of external objects.
My point is that this "list" will never be satisfied; it will either never end, or end in a paradox, with one constraint conflicting with another. In short, we can never know what "knowledge of water" is.
This is different than knowing what water is, by the way.