Students who take water into the examination hall may improve their grades, a study of 447 people found.
Controlling for ability from previous coursework results, researchers found those with water scored an average of 5% higher than those without.
The study, from the universities of East London and Westminster, also noted that older students were more likely to bring in water to exam halls.
It says the findings have implications for exam policies on access to drinks.
The researchers observed 447 psychology students at the University of East London - 71 were in their foundation year, 225 were first-years and 151 were in their second year.