Hi- just back from the Magic and hope this helps:
I brought a soft-sided cooler and never used it. We planned to have a lunch packed for our day in Cozumel, but were told by Monika in concierge that that port doesn't allow food to be brought off the ship. So we never did use the cooler.
As far as snacks, we had tons (some were part of the wonderful Fathers Day gift basket from Shirley at The Perfect Gift) and half of them were left uneaten. Believe me when I tell you there is so much food on board you won't be hankering for snacks-not even chocolate. (If I ate one more chocolate dessert I would barf!)
As for bottled water, skip it and bring water bottles. The water on board is nice and cold, tastes great and is free. I was all worried about the water, and again, I was worried for nothing. Even in Mexico you could buy bottled water (cheap) if you forgot to bring your own. Get the water from your stateroom or go to the drink stations. There is bottled water on the ship but it's not cheap.
Wine, beer and other alcohol are allowed on with you. My DH had a bottle as part of his gift basket, as I didn't want to travel with it. He drank only about 1/2 of it over the course of the week, and we had to leave it on board on our return as no opened bottles or food could leave the ship.
Any questions just ask!
Kathy