Raheem Sterling's first goal for Manchester City ended Watford's resistance and helped his side continue their 100% winning start to the season.
The Hornets dug in well for the first half but the game changed 88 seconds after the interval when Sterling slotted home Bacary Sagna's cross.
Fernandinho made it 2-0 with a fierce finish from David Silva's pass.
Watford did not manage a single shot on target as City saw out the win to move three points clear at the top.