Probably not 100% clean, but some rinsing won't hurt right?
On the more serious side: As long as you're not buying it from vending machines,
a pack of 12 / 24 / 36 water isn't that expensive at most grocery stores.
$3-5 at most
Alternatively, you could walk into a recycling centre, and see if anyone will sell it
to you for $0.05 each X 12 for a dozen.