They use saline instead of pure water because it's isotonic with respect to serum/plasma. i.e. you're not going to go into electrolyte imbalance if a lot of it

They use saline instead of pure water because it's isotonic with respect to serum/plasma. i.e. you're not going to go into electrolyte imbalance if a lot of it is put into your blood. Normal saline is about one-quarter the concentration of sea water, by the way.