We'd use 'a little', or refer to the vessel of the water instead:
Kan jeg få litt vann?
May I have some (lit: a little) water?
Kan jeg få et glass med vann?
May I have a glass of water?
Beware that we also use 'vann' as a countable noun, to mean 'lake', so if you ever see "noen vann" written it means "some lakes".