Can you bring in a backpack with water bottles in it?

As far as water in the bottles, one (store bought) per person, if sealed.

Can you take a water bag?

As several others have said, it must be factory sealed through the metal detectors and bag check. If you want to bring your own non-factory sealed water bottles, e.g. a nalgene bottle or such, you can get them refilled in the park for free. Lastly any park cups or water bottles not factory sealed must be empty upon entry or re-entry.

As long as you have nothing bad in the bag yeah should be fine.

As long as it's unopened and not tampered with

Yes

Must be sealed

I believe so they will also give you free water at any drink/ food location this includes filling a reusable bottle that you bring

Thank you all

Bring empty water bottles. Also, water is free at the stands.

Yes you can! Water from the fountains are.... Ahhh okay tasting water.