It would depend on several things like which resort, how many rooms, rack rate for your room, amount of room discount, how many people and what ages, how long are you staying, how much do you eat...etc. Here's what I did to decide if it's worth it. Figure out what your room discount per day would be and then decide if you think you can feed yourself (and/or family if applicable) for that amount of savings per day. For example, if you get a value room that is $95/ day, at 15% discount for room only rate, that's only $14.25/day savings. That means, that if you take the room discount, you would have to feed yourself and/or family for only $14.25 to make the room discount worth it. That would be pretty hard. Now, if you're going deluxe resort and your savings is a LOT per day, obviously that would be the better option over free dining. So, list all that info. Basically, you need to figure out...how much does the room save you...how much does the dining save you (and how much you think you'll spend per day in food)...and then decide which is the better deal. I can tell you this...buying the QSDP does not save you much. BUT if it's free, it is a good value, especially if you have a family. We have lots of kids. The free dining plan is a way better value for us. We have 2 rooms at a value, which would give us a maximum of $30 discount per day total (not per room). There's NO way I could feed 9 people for $30 at WDW.
As for the QS dessert...do you mean the snack? or the dessert that comes with your meal? You can use your snack credit to get water. You can also get a water as your beverage with your meal (I think), but you can't switch out your dessert that's included in the meal for a bottle of water. At least, I don't think so. however, you can get free ice and water in a cup at all CS places in the park, even if you're not ordering anything. or so I've been told, lol. We usually bring our own into the park. We stop at
Walmart and buy a couple of cases of water. You can also get it delivered from some places to your room.