Does anyone know when Unity will have a physical engine for fluids?
nVidia Physx already has uFlex that provides fluids, however that is Windows and nVidia only thing. Therefore, Unity cannot include it in the engine directly.
Even after that, Unity actually needs to implement GPU Phsicys for nVidia Physx.
However, asset store has many alternatives to fluid physics and you can use any one of them.
Also, you can follow Unity Roadmap: Platform roadmaps | Unity
There is currently no 3D fluid physics in the roadmap, so probably not in the near future.