To me "I wanna drink some water." is more natural.
With a "little" it sounds to me that there´s some kind of limiting situation (aka "limiting or negative meaning" topic on the subject).
And as pointed out by Andrezza, some would mean generally speaking "a little of water".
Explaining further: If I said "I wanna drink a little water", to me it suggests that I a so critical state that a little water would do. Perhaps I am on IV feedings and "a little" would mean wonders for me!
To, others drinks, liquor/whiskey for example I have seen examples with "little" on the web.
Even so, there aren´t many of those, compared with "some".