At 9 a.m. on Thursday, hospital staff noticed the water pressure was low.
The borough told them it was because of a water main break on 19th Avenue near Remembrance Street.
In all, 27 surgeries have been cancelled.
The hospital, which is part of the McGill University Health Centre, said Thursday afternoon the water pressure is slowly improving.
"We are noticing internally a rising pressure, so that's positive news," said Richard Fahey, spokesman for the MUHC.
In a statement, the borough said crews were working to fix the water main break and water pressure was expected to gradually return to normal.