We freeze our water bottles (one for each) 2/3 full, and 2 unfrozen and put in the backpack. The frozen bottles serve as ice for our fruit, sandwiches, etc. And the unfrozen water we put in the frozen bottles to have ice water. By the end of the day, we've had plenty of ice cold water, kept our food cold and don't have to worry about leakage.
The backpack is pretty heavy for the 1st hour, but then gets lighter and worth it's weight in gold. This is one of the biggest money saving tips I have and use 100% of the time. Think of the last time you had real ice water in Disney...they do a good job of keeping it cold, but never as cold as ours