Welcome to HCM,
Presence of bacteria and RBCs indicate Urinary tract infection
Your doctor is right in saying as it can be easily treated by urinary specific antibiotics, also flush system by taking plenty of fluid and water.
If required a culture and sensitivity can be ordered and infection can be eradicated, this has nothing to do with bottled water.
That is not the source but if you get repeated infection then any obstruction to urinary tract should be ruled out.
Please also take care of Hygiene as many times that is the cause.
If you are sexually active then also need to take care of the source.