And while Darius Henderson's knockout first-half spot-kick gave the happy Hornets three precious points in their own quest to survive, Marlon Harewood's costly miss, on the hour, has surely left the East Enders down and out with just 11 games remaining.
Bottom-placed Watford - with just two Premiership victories all season - came into this crucial clash five points behind 18th-placed West Ham, who, in turn, were five points adrift of safety.
But, unbeaten against the Hammers in their two previous clashes this season, the Hornets could claim the psychological edge over Alan Curbishley's men thanks to a 1-1 draw at Vicarage Road back in August and their FA Cup fourth round victory at Upton Park just a fortnight ago.
After seeing his team lose at home to Bolton Wanderers last weekend, Aidy Boothroyd had made a trio of changes, handing starts to Tommy Smith, Steve Kabba and Johan Cavalli and with just a dozen minutes on the clock, the Hornets' hoodoo over the Hammers duly continued.
This time, Henderson did not need too much encouragement to hit the turf when he was tugged back by Anton Ferdinand as the pair chased an innocuous up and under into the Hammers' area and, after dusting himself down, the Watford striker coolly tucked his penalty into the bottom left-hand corner.
Curbishley had made a frantic five-man shuffle of his pack following last week's defeat at Aston Villa and alongside Robert Green, Lucas Neill, Matthew Etherington and Harewood, that Watford opener signalled a miserable return to action for Ferdinand.
Having been dealt the early blow of Henderson's first goal of the season, the Hammers' boss received another awful hand on the half-hour, when Neill again fell awkwardly on his fragile ankle and hobbled away, having failed to complete either of his two matches for his new club.
With Watford content to get yellow shirts behind the ball and play on the break, it all added up to miserable first half for the East Enders, who mustered efforts from Nigel Quashie, Calum Davenport,and Bobby Zamora without seriously threatening Ben Foster.
But right on the stroke of half-time, the England new boy showed just why Steve McClaren had handed him the gloves to face Spain on Wednesday evening, when he brilliantly parried aside Yossi Benayoun's venomous 15-yarder following a quickly-taken free-kick.
With the home fans in the sell-out crowd of 34,625 finally raising a cheer, there was an equally rapturous reception for substitute Carlos Tevez, who emerged for the restart and quickly sent a 25-yard free-kick curling inches over Foster's right-hand angle.
And after the Watford keeper again thwarted Benayoun following the Israeli's impish dribble, the teasing and tormenting Tevez raced on to Harewood's flick-on and sent a twisting volley goalwards, only for ex-Hammer Malky Mackay to knee the ball off the line.
As the hour mark approached, West Ham finally looked set to level when Gavin Mahon hauled down the escaping Nigel Reo-Coker as the Hammers' skipper charged into the Watford area.
But whereas Henderson had found the target from the spot, Harewood whipped his penalty wide of both Foster and his left-hand post, to the delight of the visiting fans and the dejection of the East Enders.
That was the closest that Curbishley's men were destined to come to rescuing a must-win match and as Tevez faded and his team-mates ran out of impetus and ideas, the hapless Hammers stuttered to the final whistle amidst chants of 'you're not fit to wear the shirt'.