We usually take our own water and snacks. I carry it them a small collapsable cooler. I think the rule is that any cooler that comes into the parks must fit in a locker.
We always bring drinks and snacks. For example: Bottled water (then refill the rest of the day) and breakfast snacks.
My only suggestion is if you bring ALOT, get a locker! If you carry a BIG back pack, its hard to find a safe place to leave it when you ride the rides. Plus it will be heavy. If you require a stroller, you can keep some stuff in there.
We usually only bring a few things to get us through to lunch (plus the water).
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