You maintain that there are two notions here: 'water' and 'the taste of water'.
You assert that they are different things, yet, our only empirical basis for the former, is the latter. ...
Last sentence is not true. (Thus your arguement falls apart.) Experiences in dreams, like fear, red fire, taste of cake, etc. are a quick counter examples showing this is false but more interesting experiences in wide awake persons also refute this statement.
Normally people only experience the taste of water when there is water in their mouth, but this is not always the case. Some people have abnormal experience that even cross senses. This malady is called "synaesthesia." For examples etc. See:
http://www.scientificblogging.com/news_releases/synaesthesia_smelling_a_sound_or_hearing_a_color. Also wiki has a reasonably good article on synaesthesia.
More common (at least a few percent of humans, perhaps >10%) is abnormal sensory experience within one sense via transferences. For example tomato and orange juice may produce the same taste or after a stroke these once different taste experiences (of tomatoes and orange) are indistinguishable.
The most typical cross senses experiences are between smell and tastes (This is no doubt due to fact in our evolutionary history smell and taste were one sense.) For example, when a snake is waving his tongue around in the air a cm from his lips he is having experiences with it related to the rare molecules in the air with one integrated "smell/taste" sense. It makes more sense in most primitive organisms to call this single sense the "chemical sense" and that is often done.
The really interesting cases are the crossed sense experiences. There are people for whom a narrow range of sound waves will make then experience "blue color" and some other narrow range of sound waves will make them experience green color.
Technically water (pure H2O) has no taste. There are some other chemical compounds that are without tastes for a significant fraction of humans and yet have a very distinct and strong taste for the remainder.* One is often used in taste research studies, to initially sort these people into two groups, but I forget its name and chemical formula.
Sound, color, taste are all experiences and in most individuals provoked by the same stimuli, but not all. This is another reason for clearly distinguishing between the agent which provokes an experience and the experience itself.
Also note that specific experiences can occur without any agent provoking it. Unfortunately many people hear a buzzing or some other sound all the time. This unfortunately common malady is called "Tinnitus." (No agent is provoking the sounds heard.) Again: in dreams ALL of the sensory experiences can occur in normal people without any provoking agent.
Quite a few people have a taste experience without having the corresponding agent in their mouth.
Summary:
Sound, color, taste are all experiences and can occur in the absence of any provoking agent. Only ignorance of these facts allows one to make an identity between the experience and the provoking agent. I.e. it is pure ignorance that allows one to call the taste of water, "water," especially or "doubly ignorant" in this case as pure water is tasteless.
An experience and its normally associated provoking agent are not the same thing.
To make this all more "thread related", note that we only know the world via out sensory experience it provokes and every one of the sense can be shown to miss-inform us at times. I.e. all senses can make us believe that the agent normally provoking the experience we are having exists with the characteristic we sense. A common example is to place hand, which has been a few minutes in ice water, into a bucket of room temperature water. (Experiencing that water as "warm.") The other hand, which has been in hot water, will experience that same room temperature water as "cold." Another "false experience" is of a yellow spot on a uniformly white wall after looking at a well illuminate blue spot for a few minutes. Again all of the senses can give false reports about conditions in the "real world." Grom this alone, it should be clear that one of our sensory experiences can be identical with the agent normally associated with the experience.
Read large type bold text above again and note that no agent may even exist to provoke the experience or the experience that is provoked by an agent is very different than what is normally associated with the experience. (White wall provoking yellow spot, blue color by an audio frequency band, etc.)
--------------------
*Humans also divide into two distinct groups in their color sense. (I am not speaking of the "color blind" who lack one of the three wavelength selective cells in their retina, but of people with "normal color sense.") It is only a small difference, never noticed without careful measurements. There are two different forms of "green rodopsine" molecule with slightly different peaks in the absorption curves. I.e. some people have DNA which codes for one and others have DNA which codes for the other. The difference has such little functional effect that evoluion has not selected one form only.
