It is conceivable that, given we assume "water" does not also refer to Ice or Vapour, but only to H2O in its liquid form, then it is an essential part of understanding the term.
Both Saul Kripke and Hillary Putnam, amongst others, subscribe to a notion of natural kinds (such as water) which defines them based on internal microstructural features (such as the room temperature chemical structure, H2O), but our reference to them does not depend on this. We may still refer to the water, as generations before us have, without knowing that it is H2O, but by identifying it with some basic macroscopic properties such as being colourless or liquid, and so on. So, this is a concern when determining what will be included in our definition.
If you are looking to know whether this is analytic or synthetic, let us consider this. It may be analytic, only insofar as liquidity is contained within the concept of water, but it may still be a posteriori as the discovery of something like this involves interacting with the real world (according to Kripke in Naming and Necessity). So this will be determined, I suppose, the type of knowledge contained within our definitions.
Although, with an entity like water, there has been a substantial move away from the notion of definitions. Such an approach was termed descriptivism, and the descriptivists such as Russell and Frege proposed that we could form sets of descriptions to determine how terms within a language make reference to entities. Kripke, Putnam and Burge have all moved towards the direct reference theory which posits that the meaning of a term is contained within the object that is being referred to. So, when I say "water", I am not referring, through a set of descriptions which picks out all of the things that are water. Instead, I am referring, through a chain of reference forming a network throughout our linguistic community (Burge), where the term is used to pick out a certain entity by those before you and those around you.
I hope these considerations at least point you in the right direction for further investigation.