"Vattnet" is the definite form of "vatten". "Jag vill bada i vatten" - I want to bathe in water. (Doesn't matter where it is as long as there is water) "Kan man dricka vatten?" - Is it possible to drink water? "Jag vill bada i vattnet" - I want to bathe in the water (over there, in the bathtub, in the lake, in the sea). "Kan man dricka det här vattnet?" - Is it possible to drink this water?