Ross Barkley scored a stoppage-time winner and Ben Chilwell returned to action as Chelsea ended their season on a high note.
Kai Havertz opened the scoring but Dan Gosling’s late equaliser looked like earning a point for relegated Watford.
However, Barkley, on as a substitute, headed in Reece James’ cross to restore the lead.
Chilwell, out since November with a knee injury, came on in the final minutes at Stamford Bridge, where Antonio Rudiger made his final Chelsea appearance.
The Blues went ahead when a slick move culminated in Kenedy, who was given a rare start, driving the ball across the face of goal and Havertz applying the finish from close range.
Chelsea were the better side but some of the shoddy defending they have been guilty of in recent weeks was evident again.
In the first half, Edo Kayembe dragged a shot wide of the target after Saul had given the ball away.
In the second, Cesar Azpilicueta almost scored a comical own goal.
Azpilicueta didn’t look before a back-pass to keeper Edouard Mendy, who was standing on the opposite side of the goal. The ball was heading towards the net before Thiago Silva’s intervention spared Azpilicueta’s blushes.
And when Adam Masina swung in a cross from the left, the unchallenged Gosling pounced to score.
But Chelsea quickly hit back, with James delivering from the right for Barkley to net his first – and only – goal of the season.
Chelsea: Mendy, Azpilicueta, Rudiger (Barkley 66), Silva, James, Kante, Saul, Kenedy (Sarr 59), Ziyech, Havertz, Mount (Chilwell 89).
Subs not used: Kepa, Jorginho, Lukaku, Pulisic, Loftus-Cheek, Chalobah.