rest while water fasting

Everything is individual. The less energy you have the more you need to rest in and outside of fast. Outside of fast you could do research on how to improve energy levels.
I believe you need now to follow your body's call and rest as much as you need, and this is what most fasting specialist advise on water only. You need to direct all the energy available toward cleaning and healing.
Breathing 10-15 min is a good idea - I recommend Pranayam (add the keyword English) on youtube by Baba Ramdev - you can skip the first few times the beginning.
Walking like in 10-15 minutes is a good idea too, only if you have the energy to do it. Otherwise - just rest, don't even watch or listen to anything.
I wish you a peaceful fasting,
DL