There should not be any difference in the action of Botox if sterile water was used instead of saline. However, there was probably more pain. The official Allergan recommendation is to use non-perserved normal saline. Very few injectors use this variety, preferring normal saline that has been preserved with Benzyl Alcohol. Benzyl Alcohol is a mild anesthetic, besides being a preservative.
Interestingly, I have a patient who broke out each time, I injected her with Botox. I determined she was allergic to Benzyl Alcohol. Curiously, though the non-preserved is the indicated diluent, I found it very difficult to locate this and had to find it in a hospital pharmacy. This shows very few of my colleagues are following the Allergan recommendation.
The injections would be a bit more painful because 1) the absence of Benzyl Alcohol and as Dr. Mills points out, the pH of normal saline is closer to body pH than water.
However, efficacy will not be affected.