Potable water chemical preservation
It seems to me that the risk of adding soduim benzoate, while small, is higher than adding no preservative at all. I doubt you really need any type of preservative.  If you begin with impeccably clean bottles...perhaps sanitized the way a home brewer might sanitize beer bottles (using iodine or other sanitizing product), and work cleanly, I don't see a safety risk here. Use a known clean water source.  You could also decrease your worries further by using distilled water, but the taste might not be agreeable.  It seems like the biggest potential problem is that you might grow some algae.

Now, I don't quite understand your comment above where you state that a 16oz container would contain a hundred or so servings.  If you intend on taking a swig, and re-capping the bottle...well, every time you do that you introduce bacteria into the equation. Additional potential problems arise if you were sharing that bottle with others.  This, of course, could be remedied by pouring your dose into a cup.