After a slow start the game burst into life when Bruno Fernandes was played through one on one with the Watford goalkeeper, Ben Foster, who brought him down. Fernandes dusted himself down and calmly converted the spot kick, his first goal for his new club since his protracted move from Sporting Lisbon was finally completed.
Four minutes after half-time, Watford thought they had equalised through Troy Deeney, only for VAR to rule it out for handball. Soon afterwards, it was United who doubled their advantage through Antony Martial, who scored his third goal in a week, reacting well after Foster saved his first shot, and supplying a fine finish.
The third goal arrived 15 minutes from time on the counter-attack, with Mason Greenwood and Fernandes swapping passes before the teenage striker cracked his shot in off the underside of Fosters bar.
The result leaves Watford in the bottom three, although Aston Villa in 18th place are just one point, and one goal, better than them.
Arsenal beat Everton 3 -2 in a thrilling match at the Everton. The away side were ahead as early as the first minute, as the home side failed to deal with a cross into their box, and Dominic Calvert- Lewin scored with an overhead kick. A lengthy injury delay resulted in Arsenal having to replace their left-back Sead Kolasinac, and it was his replacement, Bukayo Saka, who set up the equaliser, crossing for Eddie Nketiah to escape his marker and side-foot past Jordan Pickford in the Everton goal.
Six minutes later Arsenal were ahead, David Luis slipping a fine ball through for Pierre-Emerick Aubameyang to outpace the defence and slide home the second. Everton equalised on the stroke of half-time through Richarlison, with Arsenal again failing to deal with a set-piece, although the Gunners goalkeeper Bernd Leno will reflect he might have done better.
Many people were still making their way back to their seats when Arsenal regained the lead, Nicolas Pps cross find Aubameyang, who scored with a powerful header. That was by no means the end of the action. Leno atoned for his mistake by making a point blank save from Calvert-Lewin, and then by making a last ditch clearance as he raced out of his goal when he was the last man.
At the other end, Arsenal almost scored a fourth when hesitation in the Everton defence presented Nketiah with a chance, but his shot hit the underside of the bar. Evertons last chance fell to Calvert-Lewin when he steered a header just wide, as Arsenal clung on for the three points.