A Danny Graham brace along with goals from Henri Lansbury and Marvin Sordell did the damage against a Coventry City side who have failed to win in 11 games.
The Hornets stung twice in two minutes to take control of the game.
First a 16th minute goalmouth scramble led to a clumsy Leon Barnett challenge on Graham.
Referee Mark Halsey, taking charge of his first Championship match since being in remission from throat cancer, pointed to the spot and Graham stepped up to smash the ball into the top right corner.
Coventry then failed to concentrate from the restart and when Clinton Morrison's wayward pass went to Michael Bryan, he found Lansbury in space 30 yards from goal and the Arsenal loanee fired a superb low shot into the bottom corner.
The Sky Blues improved somewhat after the second goal and they could have pulled one back after a good move between Aron Gunnarsson and Stephen Wright saw the latter cross for Richard Wood six-yards from goal, but former Birmingham defender Martin Taylor managed to block the effort.
Both sides continued to attack in the second half and Coventry produced their best move of the match in the 51st minute when good build-up play from Carl Baker and Freddy Eastwood gave Morrison a chance from the edge of the area but he shot wide.
At the other end, City keeper Keiren Westwood kept his side in the game with a superb block when he tipped over Don Cowie's shot from six-yards.
But that was the last of Coventry's resistance as Watford added two more goals in quick succession.
In the 79th minute, Jon Harley ran down the left and sent a pass towards the back post for Graham to sweep home and then, two minutes later, substitute Marvin Sordell ran through on goal to slot the ball home for a fourth.