Carlos Carvalhal enjoyed a remarkable first victory in charge of Swansea after seeing his new side come from 1-0 behind to net two very late goals.
Andre Carrillo opened the scoring for the Hornets on 11 minutes at Vicarage Road after calmly nodding the ball home from close range.
Watford thought they netted a second early on in the second period through Molla Wague but the goal was disallowed for a push. And it ended up proving vital as Jordan Ayew was on hand to tap home to level the scoring before Luciano Narsingh netted a dramatic winner in the 90th minute.