Pep Guardiola’s side cruised to victory on a wet and cold Manchester night where they could have had double figures but were let down by their own lack of finishing.
Watford made it easy for the hosts with none of the goals coming from any sort of difficult distance. Sterling was unmarked at the far post for his, Christian Kabasele tucked into his own net from close range and Aguero didn’t have to work hard for his either.
Andre Gray struck back late for the visiting Hornets but the damage has been done by the hosts.
City strike quick but visitors grow into proceedings
The writing was on the wall for the visitors from inside the very first minute. With their first move, the hosts scored their first goal as Sterling stroked home from a Sane cross.
With the rain bucketing down in Manchester, the hosts looked as if they would flood Gomes’ net at every chance but a lack of clinical finishing denied themselves a chance to double the lead pretty early on.
John Stones had a golden effort from around eight yards out after Sane jinxed into the box but the defender could only fire over with Gomes beaten.
It took another five minutes before the hosts would have their second. Kevin De Bruyne, in the starting line-up despite injury fears, played a cross into the Hornets box with Aguero waiting at the far post.
Fellow Belgian Kabasele tried to deny the hosts but ended up turning the ball into his own net.
Watford had chances as they pounced on space gifted to them by the City back line. Etienne Capoue and Gray both had chances but could fire their efforts into the arms of a grateful Ederson.
City put their foot down
As the first-half came to a close, it was the visitors beginning to grow into the game that may have concerned Guardiola as he trod in for his half-time team talk.
But, what the Spaniard said at the break worked.
His side put their foot down and tried to kill the game off only minutes after the interval. De Bruyne, Silva and Sterling all linked up well but neither could find the killer blow.
Aguero had a glorious chance when De Bruyne floated a flat cross across the six-yard box but the Argentine could only head wide.
However, just after the hour mark, on a less than vintage night Aguero would have his goal.
The hosts took a short corner, De Bruyne fired the return ball across the face of goal. As Gomes spilled the ball, Aguero was waiting to tap home from 2 yards out.
Claims of offside for the third would be quickly dismissed with one Watford marker playing the whole of the City attack, who were inside the box, onside.
Sané and Silva shine
On a wet, cold and generally dismal night at the Etihad Stadium, captain for the night David Silva and Leroy Sane shined brightest for the hosts.
The pair went to work on Daryl Janmaat all night and found little resistance from the Dutch full-back. The former Newcastle United man did have one bright spot as he stopped them in their tracks once but for the most part, he struggled.
Silva ghosted between both flanks with ease whilst Sane supported the returning Fabian Delph and Aguero as easy as you’d like.
Gray strikes late
Andre Gray finally gave the visiting Watford fans something to celebrate as he tapped home with seven minutes left to play.
The forward found himself unmarked inside the City box after good work out on the left-hand side by Andre Carrillo.
He could have doubled his tally and made the hosts sweat moments late but fired wide from outside the 18-yard-box following pressure from Nicolas Otamendi.
Despite the late pressure, City were more than comfortable on a night where they extended their lead at the top of the Premier League table to 15 points.