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.
Just 25% of the 447 students entered the exam hall with a bottle of water.
Of these, the more mature students (those in their second year of degree study) were more likely to bring in water - 31% did so compared with 21% of foundation year and first-year students.
After taking students' academic ability into account, by examining coursework grades, the researchers found foundation students who drank water could expect to see grades improved by up to 10%.
This improvement was 5% for first-year students and 2% for second years.
Across the cohort, the improvement in marks was 4.8% for water-drinking exam candidates.
The research paper said information about the importance of staying hydrated during exams should be targeted at younger students in particular.
Dr Chris Pawson, from the University of East London, said consuming water may have a physiological effect on thinking functions that lead to improved exam performance.
Water consumption may also alleviate anxiety, which is known to have a negative effect on exam performance, said Dr Pawson.
"Future research is needed to tease apart these explanations, but whatever the explanation it is clear that students should endeavour to stay hydrated with water during exams," he said.
Dr Mark Gardner, from the University of Westminster, told the BBC: "We find the results exciting in that they translate findings from the laboratory to real world settings like this.
"Also, supplementing with water is a really cheap way students and educators can help get better results.
"There are also implications for policy makers in terms of the availability of water on campuses."