You have some very difficult problems here.
The issue is that manipulating 3D noise is incredibly difficult when compared to 2D noise. Creating 3D islands which taper outwards at the bottom is very difficult and something I only achieve with luck rather than skill. I would recommend you try lowering the y frequency on the 3D noise to achieve this kind of affect.
You will not be able to create water on top of your floating islands. We have thought about creating a system to spawn water naturally at varying heights but have not created it yet . Currently I don’t believe we support placing water blocks in prefabs. If we did, prefabs would be the best way to achieve this. I don’t think it would be much work for us to add water and lava blocks to prefabs. We could do this in the future.
Creating lakes is relatively easy. I would take a look at the 2D ditches node, this might be a good start. If not try experimenting with this node setup:
Worley can produce round areas. Using distortion we can create blob like areas (lakes).
Creating mountains around lakes can be complecated. There is a lot you can do with 2D noise to try and get this effect. I would try using the “Rivers” node with negative river depth to create mountains. This should with the right settings also produce lower lake like areas.