Michail Antonio, scorer of all four goals in the 4-0 win over Norwich City last time out, opened the scoring as he fired through Ben Foster’s legs in the sixth minute.
Four minutes later, Tomas Soucek’s header from Jarrod Bowen’s right-wing cross made it 2-0.
Declan Rice’s swerving strike from 30 yards put the Hammers in complete command, though Troy Deeney pulled a goal back four minutes into the second half after Abdoulaye Doucoure hit the post.
However, the Hornets never looked like completing a comeback as West Ham moved six points clear of the bottom three. Watford are just three points above the drop zone with two games to play.