Struggling Birmingham City were left ruing a defensive lapse that ruined an otherwise much-improved performance as Emmanuel Dennis' first-half goal gave Tom Cleverley a win in his first game as Watford interim head coach.
But the Hornets did not give their temporary boss much to smile about against a perky Blues side who were the better throughout and created a glut of good opportunities.
Ethan Laird hit the bar and Koji Kiyoshi and Juninho Bacuna went close before Watford were handed the lead.
Collecting the ball from a throw, Emmanuel Aiwu dithered on the edge of his area and was robbed by Dennis who then gave John Ruddy no chance.