The best is not to take supplements of any kind during the water only fast.
No supplements are designed for water fasting, they are designed for people who consume foods.
After all, all those supplements are industrial products, and may actually cause much more harm than good.
Just a fact that you are not eating will be enough to increase your immunity to the point where your body can beat just about any infection.
Most infections inside body, like UTI, are caused by the consumption of processed carbohydrates (sugar , packaged juices, sodas ), or unprocessed carbohydrates (grains , fruit) that your body has problem tolerating.