In my own opinion, water and lava both have completely natures. I think I understand what White Knight is saying, but honestly I think the only thing lava and water have is that both are liquid. Lava is thick, heavy, highly viscous and flows slowly. Water being the opposite with its fast movements and ability to move through just about any porous surface. Water and earth in my own opinion do indeed share very similar fundamentals in terms of phase changing (besides earth having an gas form, unless you count sand) and they do have SIMILAR movements; but I don't think they can be bent in the exact same way. Water flows alot faster and easier than lava can, allowing it to be streamed, pressurized, and shaped to the user's desire. Lava (as far as we know) CAN be streamed, but in a much slower and less aggressive way. I also want to add me fact that lava is indeed heavier than water, restricting it from sharp turns and angles (I assume). In a conclusion (sorry if it's confusing just sorta thinking as I go) I'll have to say that while water and lava are bent similar, the properties and nature in which lava works restricts it from being ENTIRELY like Waterbending. It's funny cause ice bending and earth bending share my same argument, just because they are the same state doesn't mean they share the properties to make them clones.