If you're on vacation and want to spend $3.50 on a bottle of water, be my guest. But I never buy water at the parks.
It's a waste of money, especially because you're allowed to bring your own bottles in.
The old trick of the trade is to ask for a water cup at any quick-service location, but frankly, I don't like the taste of that water. Instead, I purchase a few cases of water on my way from the airport and carry them in a backpack around the parks.
It saves me money and keeps me hydrated in the Florida heat.