To add to what stated, Flex and WaveWorks could potentially become plugins at some stage, although not sure if the liquid/fluid rendering could be placed into a plugin effectively since it ties in with particle rendering. VXGI will probably never become a plugin due to the tie in’s to the rendering pipeline and post processing pipeline. But I don’t expect anyone to start working on a plugin version of any of the modules until they are more mature. So for now your best bet is to download the NVIDIA branch and get compiling.
But then again, if the only thing that actually interests you is the fluid/liquid simulation, you might as well wait, since you can simulate fluid currently, it does not render as fluid.