Its really really difficult to render water is a very non-viscous liquid. You can see from some of the shots where the water flows into pools and the white texture on top that it swirls around so they are using a fluid dynamics bit of math. I saw that they are eroding some of the land but I didn't see any sediment build up. I saw the lava just get to the water but then it cut to another scene and I would have liked to know what they did with that. Its hard to know whether its really that fast because they are calculating the land on such low res lumps. But its neat to be able to do anything like that in real time.