Javier Hernandez continued his inspired run of form, scoring his third goal in four Premier League games, while Marko Arnautovic marked his return to the starting eleven with a goal.
New signing Patrice Evra was sat on the bench for the Hammers with Arnautovic, Michail Antonio and Pablo Zabaleta returning, while rather unsurprisingly Watford opted for the same starting eleven that thumped Chelsea on Monday.
The Hornets almost took an early lead from a clever corner. Gerard Deulofeu rolled the ball for Marvin Zeegelaar to run onto and whip an out-swinging ball onto the head of Adrian Mariappa but the defenders header was saved by Adrian.
The Hammers then replied themselves through Arnautovic's dazzling feet. He made his way into the penalty area past three Watford players and when he was eventually brought to a halt, the ball fell kindly to Joao Mario but Orestis Karnezis denied him of his first goal in a claret and blue shirt.
Hernandez thought he had continued his rich vein of goals coring form in the 31st minute but his header from Joao Mario's free-kick was chalked off for offside. Replays showed there was very little in it.
Arnautovic then had a golden chance to open the scoring but squandered it from point blank range. He latched onto a header down into the six yard box but put his shot straight at Karnezis. Chicharito saved his blushed seconds later though.
Antonio did tremendously well to beat his man down the left wing and hang the ball up to the back post for Hernandez to head home in the 38th minute. This time it did count despite Watford protests after a coming together with Zeegelaar in the process.
After the restart, Deulofeu hit a dipping free-kick but Adrian was able to palm it away. West Ham were beginning to assert themselves as the dominant force in the game and Karnezis - while not seriously tested - was becoming busier.
In the 78th minute, they finally doubled their lead as Arnautovic marked his return with the easiest goal he'll score all season. Watford attempted to clear the ball but it ricocheted off Aaron Cresswell and fell to Arnautovic in the six yard box who emphatically finished past Karnezis.
The win moves West Ham up to 12th place, level on points (30) with Watford in 11th.