Understanding what facilitates and hinders water drinking is crucial to inform interventions for preventing underhydration. Using the Situated Assessment Method², we extended previous research by examining what influences water drinking in daily life. We studied 213 UK adults, assessing 13 potential predictors (e.g. thirst, availability of other drinks) of their typical water intake across 10 everyday situations (e.g. during work, dinner). Participants then reported their actual water intake in these situations over three alternating days during a 1-week follow-up. We evaluated the variability of water intake and its influences across individuals and situations and the prospective relationship between these influences and water intake. The 13 identified predictors explained substantial proportions of variation in water intake. Factors like habitualness (e.g. subjective effort), self-relevance (e.g. health consciousness) and immediate feedback (e.g. taste) were positively associated with water intake. However, the influence of these factors varied significantly across individuals and situations. Our results suggest that various interrelated predictors facilitate and hinder water drinking behaviour, emphasising the importance of using comprehensive behaviour theories to inform research in this domain. They also align with growing evidence that reward may regulate habitual behaviour.