New boss Moyes' first game in charge ended in more disappointment for the Hammers as goals from Will Hughes and Richarlison got Watford back to winning ways.
The Hornets looked ripe for beating on the back of a three-match losing streak and with boss Marco Silva linked with the vacancy at Everton.
Yet although Moyes knew he was inheriting the worst defence in the top flight, he will surely not have expected them to capitulate as meekly.
READ MORE: Watford v West Ham - how the match unfolded
Moyes had talked of taking West Ham back to basics on the training ground, a method which became apparent in the opening five seconds of the match.
Andy Carroll took the kick-off by passing back to Angelo Ogbonna and then charging forward to get on the end of the defender's punt forward.
Hornets debutant Marvin Zeegelaar was the unfortunate recipient of Carroll's unintentional elbow and required four minutes of treatment.
However, Zeegelaar was eventually on his feet and played a key role as Watford opened the scoring in the 11th minute.
The Dutch wideman crossed towards Hughes, whose initial attempt at the far post was only half-cleared by Aaron Cresswell.
Andre Gray swung at the rebound which deflected back to Hughes, who swept in his first goal for the Hornets on only his second Premier League start.
It seemed West Ham had slipped straight back into the chaotic displays of last few weeks under previous boss Slaven Bilic.
Richarlison was allowed to saunter into the penalty area and force Joe Hart into a save with Kiko Femenia drilling the rebound, via a deflection, inches wide.
Yet somehow the Hammers twice almost forced an equaliser shortly before the interval.
First Mark Noble put Cheikhou Kouyate through but the midfielder's shot was too close to Heurelho Gomes.
And in stoppage time Pablo Zabaleta's cross was met by Marko Arnautovic, who was denied by a stunning double save from Gomes at point-blank range.
Early in the second half Kouyate blew another golden opportunity, blazing over from 12 yards out from Arnautovic's square ball.
Watford sensed a second would kill off West Ham and Hart pulled off a fine save to keep out Adrian Mariappa's header on the hour.
But moments later Watford did double their lead, Hughes feeding Richarlison who had the freedom of Vicarage Road to dart into before firing into the bottom corner.
West Ham knew their luck was out when Manuel Lanzini's shot was cleared off the line, possibly with the help of an arm, by Christian Kabasele.
With chants of 'sack the board' and 'you're not fit to wear the shirt' booming out from the away end, it was not a happy start for Moyes.