Some kind of antialiasing/multisampling

[Wumpf]

maybe it's time to try temporal techniques.
I know, water is splashy and everything, how should that work out? But I reckon "still quite well" and I never tried it.
Note that we also know the motion vectors of the fluid at excruciating detail!
It could give a more unified path than fixing sampling issues one by one (like refraction,reflection, etc. .... which still needs to be looked into I suppose) and opens other possibilities for accumulating samples. Also, it makes high quality renderings easy as it provides the framework for super sampling (i.e. "just stare at every image longer, it will get better over time")