Water only fast is the best way to go. If you were to do any other juice fast that will be good too, as long as you are consuming different type of juices everyday. If you are getting coconut water from supermarket that is not a good idea. These juices are all cooked and include other ingredients. If you were to drink fresh coconut juice it will be incovinient and rather expensive.
I also think that one can go in a starvation mode with eating partially or very limited. Also your body will not go in ketosis and will have lesser detox.
Emptiness feeling can be phyiscal or mental. If you want to be successful in doing longer fasts, build your nutritional reserves by eating raw vegan diet and then do a fast taking one day at a time.