The danger isn’t in the parts per billion, but in the actual amount of mercury. According to the CDC, the recommended adult dose for the Influenza Vaccine is 0.5mL, or 1/2000 of a liter (about a quart). This means the amount of mercury in the vaccine is 2.55*10^-10 liters. The smallest imperial liquid volume measure is the minim, and you’d need about a fifth of a million of these to make up one (you can verify that here).
The minimum acceptable concentration is water is a lot lower, because people drink a lot more that 0.5mL (about a thousandth of a pint) in a year. To get the same amount of mercury from maximally contaminated water, you’d have to drink 2.55*10^-10/(2^-9), or a little over a tenth of a liter. Assuming people drink the recommended 8 cups a day, they’ll get nearly twenty times that amount in a day.
h/t David Burkhead