The same topic is being discussed in another thread. No one seems to be able to find an actual policy statement regarding the prohibition of plastic bottles. While Celebrity does use glass and metal bottles for water, they do serve drinks in plastic bottles on the ship, notably vitamin water as one example.
If anyone can find a statement in the FAQ or website or E docs that says that you are not allowed to bring plastic bottles for sure, it would be appreciated it if you could post a link.