Read this tutorial:
http://www.emanueleferonato.com/2012/05/16/simulating-mudslime-with-box2d-bitmaps-and-filters/It is in flash, so it won’t work with cocos2dx, but it is a good reference and shows you a lot of good ideas. Fluids aren’t that easy to implement. The box2d part isn’t that hard, but the displaying of the fluids is the hard part. It isn’t one of those things that is normally shipped with stuff, otherwise you would see more games with it. There is are some discussions on the cocos2d-iphone forum that might be good for reference.
And if you are into math, here is a really good place to find out how to implement fluids:
In the end, your first question is answered with, “how far are you willing to go to get it?”