It is not waterproof, but splash proof based on Jawbone's website. Below is an excerpt from there website's FAQ.
The UP3™ band is rain, splash and sweat-resistant. However you should remove your UP3™ before showering, swimming, surfing or exposure to other extreme conditions and activities like saunas and steam rooms. Do not submerge your UP3™ in liquids, including hot tubs or baths, at any time.