I cannot over stress the importance of having enough water to drink while you are at Disney.
I recommend to drink 2x the amount you drink at home. It is probalby going to be hot and humid while you are there. It just makes you feel better which is helpful.
I had a vacation where 2 members of my party thought they were drinking enough (yes, I was nudging them) but they were night. By early afternoon they had upset stomaches and were just not feeling well. We had to cancel our dining reservation for dinner (at my favorite restaurant!).
Make sure to drink lots of water. Bring water, buy water, ask for water at any counter service, just drink lots of water.