I would suggest to you to follow this video from Ryan Brucks (from Epic) where you can mix the idea of filling the ball with small balls and using distance fields to merge the ones inside. With some material tweaks you could simulate movement from them (as they are merged) which will mimic at some point the water effect. I know this is like a hack solution, but it has great performance, so the number of balls inside could be dozens, and I think that with just physics responding to the movement you will likely to have a liquid motion since they are merged by the technique he explains. Let me know your findings!
