Water doesn't have a large Ka (acid dissociation constant), hence the equilibrium of
H2O ⇌ H+ + OH-
lies on the far left.
therefore a container of water wouldn't have many H+ and OH- ions floating about.
There WOULD be a very very very small concentration and H+ and OH- in it, since a value of Ka means that it does ionise in itself, but on the whole there would be MORE water molecules floating about. This is why water acts as a very poor buffer.
You need the acid AND the conjugate base (salt) for a buffer to work.