The movie's director Darren Aronofsky did not want anybody to drink water from plastic bottles in order to encourage a pro-environment atmosphere during the filming, which led Watson, 23, consume stagnant water out of thirst, reported Daily Mirror.
"Because of the storm, we ended up shooting most of our scenes between 4am and 7am - and at that time I never function well. Because the film has a pro-environment message, Darren didn't want anyone drinking from plastic water bottles on set either," Watson said.
"Everything we used had to be recycled or recyclable. Having no water bottles on set at five in the morning - when you're exhausted and delirious - wasn't ideal. Was so tired one morning I picked up a mug from my trailer and drank some stagnant water that had been there for the duration - so three months. I was so ill," she added.