What should happen if you gave the runner pure water in a iv?

The main problem with injecting pure water is haemolysis. As someone has stated, the difference in the concentration of salts inside the cells and the plasma outside (which has been diluted by the injection) will cause the red cells to swell with water and burst. Probably isn't a major issue if only a small amount is injected, but can release large quantities of potassium into the blood stream which isn't always a good idea (have no idea how much would be needed for it to be a problem, I'm sure someone has worked it out) 

For this reason, injected fluids are normally salt or sugar solutions which are comparable with plasma in terms of osmolality. "Normal saline" is a 0.9% sodium chloride solution which is close enough in terms of osmalality to work.