We describe Hydroprompt, a prototype for sensing and motivating healthy water intake in work environments. In a 3-week field deployment of Hydroprompt, we evaluate the effectiveness of three approaches to behavior change: historical information enabling users to compare their water intake lev- els across different times of day and days of week, implicit feedback providing subtle cues to users on the current hydration levels, and explicit prompting at- tempting to remind participants when hydration falls below acceptable levels or when substantial amount of time has elapsed since the last sip.