Yeah, the only things that actually move are doors, buttons, levers, and small boats and minecarts etc. Nothing at a big scale, and they're mostly moving on axis points.. It would likely be possible to group certain blocks together to get them to move on an axis, like say, if you build something like a waterwheel and have it turn on an axis and be powered by a waterfall. Robert's right though. These kind of things require for objects to be defined. To make things like this work, you'd have to select block groups and define them as either terrain or vehicle or moving object, etc. Something that the engine just doesn't do.. I would love to be able to have larger scale boats similar to the small ones though, as that might be possible.
Or, I just thought of something. I think we've been complicating things. We're all thinking in terms of terrain; moveable terrain. The way to look at it is objects. The whole gameplay is made up of objects and crafting them. We already have something with a floating property; the boat. What's to stop notch from being able to combine several boats together to get a bigger one? Say, 9 boats could make up a large boat. 6 Boats could make up a square raft. Make enough rafts and join them together to make them even bigger surface area; a surface area that could be built on. Anything built onto it would automatically become part of the raft and be thought of as one object, not several, reducing lag.