To clean water, you only need four things:
1. A big bowl, filled with the water;
2. A smaller bowl, put inside the bigger, that will get the clean water;
3. A waterproof sheet of cloth and a pebble;
4. Time in the sun, or a heat source.
Put the small bowl inside the big, cover both wiht the cloth, and put the pebble on the cloth, vertically of the small bowl. Expose the whole to the sun, or put it over your heat source.
The dirty water will evaporate, condensate on the cloth, run along it thanks to gravity to the vertical of the small bowl, and fall inside. Once the process is over, you have a big bowl full of dirt and a small bowl full of clean water.
That said, prestidigitation is already able to do things that are about as complicated, notably cleaning stuff, gathering similar stuff in a spot, altering the taste of something...