Background/Objectives: The Beverage Hydration Index (BHI) compares fluid retention of beverages relative to water based upon urine output over time. We examined the reliability of diuresis from water ingestion and its potential impact on the BHI of a sugar-free energy drink with electrolytes (CAF). Methods: Twenty-four adults completed two trials consuming either one liter of water or CAF over 30 min with urine output measured over 4 h. In a sub-group (n = 15), water trials were repeated twice (~6 months apart) to compare the impact on BHI of CAF. Results: Urine mass was not different (p = 0.46) between repeated water trials; however, intraclass correlations indicated poor reliability (r = 0.11, r = 0.31) at 2 and 4 h. Mean (±SD) BHI of CAF based upon two water trials did not differ (p = 0.44) at 2 h (0.90 ± 0.17 versus 0.97 ± 0.22) and 4 h (0.99 ± 0.18 versus 1.01 ± 0.21). The BHI for CAF at 60 min (0.75 ± 0.21; 0.83 ± 0.23) was the only time point lower (p < 0.001, p = 0.008, respectively) than a BHI of 1.0 (water control). The BHI of CAF for all participants had similar results: no difference between CAF and water from 2 to 4 h. Conclusions: Although the mean diuretic action of water ingestion appeared similar across repeated trials, the reproducibility of diuresis within individuals was poor. Despite low reliability, the impact on establishing the BHI of CAF was not significant. CAF provided similar hydration benefits as water from 2 to 4 h (varying by 1–7%) despite increased diuresis only in the first hour, suggesting that a sugar-free electrolyte beverage containing caffeine supports long-lasting hydration. Moreover, the establishment of BHI metrics is robust to interference from potential differences in individuals’ pre-test free-living conditions.