Consistent hydration is paramount for health and well-being. For knowledge workers, it also has an impact on overall productivity. Unfortunately, many knowledge workers experience insufficient hydration during busy working hours. We conducted interviews (N = 10) to investigate the causes of inconsistent hydration during work and explore the expectations of knowledge workers regarding intervention systems to improve their hydration habits. Based on the results of the interview study we designed the H2Office system, a comprehensive approach comprising a water gauge and a smartwatch app. A comparative field study over 10 days (N = 7) of the H2Office system shows its effectiveness in motivating knowledge workers to increase their water intake substantially (by 61%). The combination of auditory and vibration intervention effectively reminded them without causing any work distractions. In the evaluation of H2Office we identify detailed water consumption patterns of users concerning the time of day, sip interval, and sip quantity. Lastly, we found that knowledge workers desired a holistic approach (tracking consumption of other beverages) for delivering hydration interventions.