Since it's a cactus, it's a hardy plant which makes it very adaptable to different conditions, including water type. So tap water works just fine.
However, if you live in an area with hard water, you'll have to put in a little more effort in your watering routine. That is, you'll need to soften the water or run it through a filtration system before using it to water your plant.
Here's the deal: hard water contains extra minerals that may not be good for your Rhipsalis. To avoid potential problems, stick to bottled or rainwater (if available). Otherwise, always filter your tap water.