Some textbooks on Russian grammar say that the partitive genitive of a noun appears only as the object of a verb, never as the subject. It appears to me that воды in the above expression would be the subject. If so, it should be replaced by вода grammatically. But воды seems more frequently used. Why?
In the phrase мне хочется воды, grammatically, there is a zero subject, one verb and two objects.
It's an impersonal phrase, which is similar to English impersonal constructs like "it seems to me", "it occurs to me" etc, except that phrases like that have a zero subject in Russian (as opposed to English, which has an actual subject, "it").
So grammatically, both мне and воды are objects to хочется.
Note that object, subject and verb are purely grammatical terms, which are not to be confused with thematic roles of agent, patient and instrument.
мне хочется воды doesn't contain any subject. It's an impersonal sentence where
воды are objects. Therefore, the rule you mentioned isn't broken in the sentence.
Regarding the sentence, there's no direct analog of such construction in English. You may consider
мне хочется as a synonym of
Мне хочется воды is exactly
*I am in a state of* thirstyness, a physiological state (that implies a psychological) corresponding to a strong need of water".
Right from the English phrase we can see why there is genetive, because "of smth" has direct corresponding with Russian Gen.
хочется is very special verb. It sets a person in a certain complex state in virtue of his perceiving.
мне чудится сон Nom мне делается операция Nom мне хочется танцев Gen мне видится образ Nom мне хочется моря, песка, солнца Gen мне слышится песня Nom
In a certain sense in
мне хочется воды there is a subject, a psychological state only I know I am in. This subject you may consider (in English, like weather verbs) as included, "It's needing; (my state of the need of ...)", where in the place of "need" can be placed smth with 25+ meanings in English.
Я хочу воды vs
мне хочется воды is an inappropriate analogous. When we say
Я хочу ... we mean, first, "I am needing smth", but
мне хочется ... is "I am feeling ... (state of “being” in the present)" and "I am aware of this state".
Я хочу is an analogous
Я делаю. Thus in
мне делается операция the noun
операция is a word in Nominative, and there is huge number of examples with such verbs. It's something that we consciously try to do, whereas a psychological state is something else.
there are ... reflexive verbs accepting objects in accusative and that
хотеться, one of them, takes an object in animate accusative or in inanimate genitive.
There is more examples
хотеться_praes + N_accu without the connection with in/animate, so it is fully literate. Proof