The Hangover star, 38, fuelled speculation he is dating the 20-year-old British beauty when the pair was spotted enjoying a day out sightseeing in Boston, Massachusetts, on Monday (18Mar13).
Cooper was first seen in public with Waterhouse last month (Feb13) when they left the ELLE Style Awards in London together in the same car.
A source tells Us Weekly magazine, "It's new but they're hanging out. He's really into her."
Cooper, who is in Boston to shoot a new movie with his Silver Linings Playbook director David O. Russell, has been single since his on/off romance with Avatar star Saldana ended shortly before Christmas (25Dec12).