VENICE, Italy — I last visited the canal-crossed city of Venice during a busy August 12 years ago, and my indelible memory is of tourists and mosquitoes swarming the city in equal numbers. Overrun by a glut of pedestrians, it seemed that Venice, which is sinking by 2 centimeters a year, would go down long before its time under the crush of trampling feet. These visions kept me from returning to the city for over a decade as I searched for quieter places to enjoy.