Richarlison struck a late winner as Watford's 2-1 victory gave them a first league success at Swansea since 1983.
Luther Blissett and John Barnes were the scorers when Watford last won on Swansea soil.
But Andre Gray and the Brazilian Richarlison were the Hornets' heroes this time after substitute Tammy Abraham appeared to have given Swansea a share of the spoils.
Abraham finished from close range after 56 minutes to cancel out Gray's first Watford goal, but Richarlison brushed aside Alfie Mawson's timid challenge in the final minute to finish off the underside of the crossbar.
Swansea had not managed an on-target attempt in three of their five league games and had not scored at the Liberty Stadium this season.
Those obvious problems in front of goal were reflected in the first half, despite the re-signed Wilfried Bony starting a Premier League game for Swansea for the first time since December 2014.
Watford were the better side in the opening period and should have been ahead before Gray produced the first dividend since leaving Burnley last month in a deal believed to be worth up to B#18.5million.
Andre Carrillo sent Kiko Femenia scampering away and the right-back pulled back a cross for Gray, but Mike van der Hoorn deflected wide.
Gray, though, did not have wait long to find the net on a ground where he scored twice for Burnley last season.
Carrillo, picking up the ball near the halfway line, burst past a couple of Swansea defenders and crossed from the right.
Sam Clucas tried to cut out the cross, but his sliding intervention only diverted the ball into the path of the lurking Gray who smashed home.
The ever-dangerous Carrillo soon broke past some half-hearted challenges before Van der Hoorn headed out his cross to Femenia.
Lukasz Fabianski should have held Femenia's volley and the Swansea goalkeeper was saved by a flagging assistant referee after Richarlison had turned the rebound Carrillo's way.
Gray almost cashed in on Mawson's dreadful attempt at a back pass, but Swansea finally mustered a moment of danger when Bony failed to convert Tom Carroll's free-kick from close range.
But Swansea boss Paul Clement had seen enough and he changed both personnel and formation at half-time.
Swansea abandoned their policy of playing three centre-backs and went with a flat back four as Roque Mesa and Abraham entered the fray to provide extra craft and power.
Abraham's impact was instantly felt as he headed over a Carroll corner before the on-loan Chelsea youngster claimed his third goal in eight appearances.
Jordan Ayew cleverly fed the ball into the box over his shoulder where Abraham missed his kick and Bony needed two attempts to stab the ball goalwards.
But Gomes failed to hold Bony's second effort and Abraham, following up as the Watford defence stood still, dispatched the loose ball into the net.
Swansea searched for the winner after that and had the majority of the play, but Richarlison's sucker punch inflicted a third straight home defeat on Clement's men.