As far as the water--I don't like the tap water there (and at home and other places I'm a tap water person), but can drink it with a meal as long as there's lots of ice. Or if a person is eating a counter serve meal they can always have their own bottled water that they've brought with them.
I don't know about a family of 6 eating for less than a $100/day. Like others have said, maybe offsite. When we are eating cheap we eat breakfast in the room (oatmeal, cereal, Nutrigrain bars, etc) and then bring some foo into the parks with us (peanut packets, string cheese, crackers, etc). We eat that for an early lunch and then eat an early dinner. Or we save that for dinner and have a sit-down lunch. Sometimes we don't bring any food with us, but then eat a sandwich in the room for dinner after returning from the parks.
That plan works ok when I'm with my sister and my youngest son, but doesn't work with my family (I have 4 kids) because my older kids expect real meals on a regular basis and my DH doesn't like grazing. When my sister and I (and my youngest) went for almost 2 weeks with our mother we ate very cheaply since our mom is the definition of cheap (meant in a most loving way

). My sister and I do like at least one good meal a day and are not opposed to spending more $.
This trip (with my 18yo DD) we'll be doing the dining plan for the first time, and it will be a totally different experience.