Acceptable levels of HCl?
pH of lemon juice and cola is around 2-3. As long as you don't go below this point, drinking should be fine.

It is different if you consider fish living in such water. pH from several ppm would not drop below 5, which is close to rainwater, but significantly lower than normal river or sea water (6-7 and 7-8 respectively). You should consult specialized literature in the case.