David Moyes failed to inspire West Ham as their Premier League woes deepened with defeat in his first match in charge at Watford.
The Moyes era got off to the shakiest of starts when the home side took the lead in the first threatening attack of the game at Vicarage Road.
West Ham failed to deal with a cross from the left and, when the ball fell to Andre Gray, his mis-hit was finished off by midfielder Will Hughes for his first Premier League goal with 11 minutes gone.
The Hammers tried to hit back but Andy Carroll directed a tame header straight at home keeper Heurelho Gomes.
After 22 minutes, with West Ham beginning to put together some better possession, Watford broke but the referee waved away penalty appeals as Richarlison stumbled under a challenge from Pablo Zabaleta.
Carroll was booked for a 27th-minute push on Richarlison, with Watford's Miguel Britos following him into the book for a clumsy foul, but the home side cleared the free kick.
As the game passed the half-hour, Watford threatened a second when Joe Hart blocked from Richarlison but the ball fell to Kiko Femenia, who fizzed his angled effort just beyond the far post.
With two minutes of the half left the Hammers thought they had equalised, only for Gomes to thwart Cheikhou Kouyate from close range before Hughes sent an ambitious volley over at the other end.
Gomes was at it again as his outstanding double save thwarted a header and follow-up shot from Marko Arnautovic in stoppage time, and there was still time for Richarlison to drag narrowly wide at the other end.
Gray fired a first-time strike wide from another Femenia cross as Watford started the second half brightly, but West Ham should have levelled when Kouyate, unmarked in the area, scooped over.
Hughes set up Abdoulaye Doucoure, whose first-time strike from 25 yards curled wide of the post with Hart rooted to the spot, and then a Hughes effort was deflected into the West Ham keeper's arms as the Hornets stepped up the pace.
As the hour approached, Gray's shot deflected wide and Hart made an excellent save when Adrian Mariappa headed a Hughes cross goalwards.
Four minutes later, Watford doubled their lead as Richarlison, played into space on the left by Hughes, cut in and shot low into the corner via the fingers of Hart, who could have done better.
Moyes brought on Diafra Sajho for Carroll as he tried to rejuvenate his side, but the Hornets were in control and West Ham suffered another blow when Arnautovic injured his wrist in a collision and was stretchered off to be replaced by Arthur Masuaku.
Manuel Lanzini almost pulled one back for Moyes' side as the game entered its final 15 minutes when his shot beat Gomes but was cleared off the line by Christian Kabasele.
But Watford saw the remaining time out comfortably and could have added a third when Richarlison fired wide.