Pure water into your vein would cause a disruption of tonicity, which would cause blood cells to swell with water and eventually rupture, leading to hemolysis. By itself hemolysis isn't really painful since it produces similar hypoxia symptoms to methemoglobinemia (the condition that SN produces), but you'd need a lot of water to dilute your blood enough to cause a deadly amount of red blood cell destruction.
The 2 biggest problems with attempting this is infiltration from either missing or blowing a vein or causing an infection, since you can't guarantee that *EVERYTHING* you use is completely sterilized of every single microbe. Overall, water injection is a really bad idea.
As for air, the only possible fatal outcome is an air embolism that lands in a vital vessel in the brain, heart or lungs. It's highly unlikely you'd be able to get a fatal amount into your system, and it's even less likely that the amount would land in a vital vessel. Even if by some chance it does manage to do so, the experience will surely be a miserable one. Again, a VERY bad idea.