After City were held to a goalless draw by Crystal Palace 48 hours earlier, prolific Raheem Sterling scored quickly and Sergio Aguero added another as City continued their unbeaten league campaign with the minimum of effort at Eastlands.
The outcome looked all but settled within 38 seconds of kick-off as Sterling gave Pep Guardiola's side the lead with the fastest goal in the Premier League this season.
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After failing to score at Palace and going the last hour without a goal in the previous game at Newcastle, Guardiola's team had played for 150 minutes without scoring, a drought by their prolific standards.
But from the first attack, the recalled David Silva found Leroy Sane wide on the left and his accurate cross was turned in by the unmarked Sterling just six yards out.
It was a stunning start and one which ensured that there would be no hangover after their winning streak came to an end in their last match of 2017.
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By the time they doubled the lead, on 13 minutes, Watford were staring at a long evening and a defeat that might match their 6-0 home loss to City in September.
The visitors contributed to their own downfall this time as Silva found Kevin De Bruyne wide on the right and his low cross was turned into his own goal by defender Christian Kabasele, with Aguero lurking.
Aguero claimed City's third, on 63 minutes, as Heurelho Gomes could only parry a fierce, low shot from De Bruyne and the Argentinian was perfectly placed to force in the rebound from six yards.
Watford's consolation came nine minutes from time when Andre Gray darted between two markers and tapped in Andre Carrillo's low cross from the left.
But it was too little, too late although, surprisingly, Marco Silva's team had managed to carve out two excellent chances of their own in a first half of City domination.
After seven minutes, Kabasele's quick free-kick sent away Gray, who outpaced the recalled John Stones, but failed to lift the ball over goalkeeper Ederson.
And the Brazilian was equally impressive, just before the half-hour, when an error by Fabian Delph, allowed Gray to race clear only for Ederson to make an excellent save.
But the rest of the goalmouth action all took place in the Hornets' area, notably in the fifth minute when Stones lifted a poor shot high over the goal from only six yards after more excellent work from Sane.
De Bruyne came close from one of his trademark free-kicks which Gomes did well to tip onto the bar, with the keeper then saving Stones' follow-up header.
Aguero's lack of touch continued after the restart when he stooped unmarked to meet a De Bruyne cross but guided his header well wide.
And De Bruyne was equally wasteful when presented with a free-kick on the byline which he drove straight into the side-netting.
Silva almost played De Bruyne clean in on goal, only for Daryl Janmaat to make a superb challenge, and Nicolas Otamendi headed wide from another testing De Bruyne cross.
The third goal eased any lingering worries about the game's outcome and it simply became a question of by how many City would win.
Aguero wasted a chance from eight yards and De Bruyne saw a 20-yard shot deflected behind.
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