If I had to, I'd take it as carry-on and just know that it's going to be a pain to drag since it will be so heavy. I'd put it in the smallest pullman suitcase that will fit the water and not put anything else in the bag to keep it as light as possible (and to protect from spillage, though that should be unlikely).
Once I got to the hotel I'd let bell services deal with it, making sure to tip them nicely. Then, on the way home I'd have an empty suitcase to fill with all the junk ...err.. priceless memento's I've bought while at WDW!
What I would really consider is to order water and other snacks and soda from a grocery delivery service and just have them bring it to the resort. You won't have to stop at the store on the way to WDW, and you won't have to lug 20 pounds of water half-way across the US, either. Plus you'll also have snacks in the room, saving you even more money!