Watford improved after the re-start, but in truth, the only way was up.
It could have been more and Watford were booed off at half-time.
Even a draw will ensure City participate in the coveted league, but it could mean they drop down to the fouth spot if Liverpool manage to beat Middlesbrough at home by three goals. They were knocked out at the hands of Monaco at the quarter-final stage.
Chelsea, with their Brazilian manager, had been trying to buy the Real Madrid striker themselves, but on the deadline day on which Dimitar Berbatov joined Manchester United from Spurs, City set a transfer record by paying £32.5m for a player who would appear for them only 41 times in an 18-monthstay.
And it was certainly not the farewell to Vicarage Road that Watford head coach Walter Mazzarri would have chosen after the announcement of his dismissal last week, when he became the eighth man in charge of team affairs to leave since the Pozzo family took over in June 2012. That left his team short of heart and spirit and it showed.
City's big win clinched third position, and with it, an automatic Champions League qualifying place.
Then it was Gomes getting in the way of a shot by Aguero.
City were razor sharp straight from the kick-off and almost ahead inside the first minute as Aguero's pressing forced Heurelho Gomes into a mistake that ended with Gabriel Jesus seeing a header cleared off the line.
The biggest Watford cheer came when Deeney went for a jog on the touchline.
The Argentina striker has endured one of his most hard seasons at City - he was even dropped at times with Guardiola wanting more from him - but he finished it with 33 goals in all competitions.
Four minutes before the interval it was stand-in right back Fernandinho's turn to find the net after Watford made a hash of a clearance, and did no better when attempting to foul in order to recover the situation.
"I want to go on until the end and the best way is going to be (playing) football with City".
The only cheer he received was when referee Jon Moss lectured him during the second half.
WHAT A MOVE: Aguero and Sane celebrate wrapping up a wonderful team goal for City's third.
City still hadn't finished punishing the Hornets and five minutes later, it was 4-0 as Fernandinho, Aguero and Gabriel Jesus battled to win possession, ending with Fernandinho wriggling past a couple of challenges and slotting the ball past Gomes.