In an uninspiring game, Deeney's header gave Palace their third successive triumph in the top flight and they now sit four points clear of the relegation zone.
Crystal Palace went into the fixture having recorded successive victories in the league for the first time since Sam Allardyce's arrival, while Watford were involved in a seven-goal thriller with Southampton last time out.
However, the opening 45 minutes in London provided very little in the way of goalmouth action, with neither goalkeeper being required to make a save.
Changes were expected from both managers but at the start of the second half, the game progressed in the same manner as the first half with only an ambitious shot from Etienne Capoue posing any threat to either goal.
However, Zaha was proving to be a threat for the home side and won Palace a number of free kicks in and around the penalty area, one of which they were able to take advantage of with a quarter of the contest remaining.
A free kick was delivered into the penalty area by Cabaye and it was the unfortunate Deeney who headed into his own net via the post.
Watford had offered very little throughout the second half but a change in mentality led to Abdoulaye Doucoure seeing a good effort saved by Hennessey in what was his first real save of the game.
Palace had opportunities to double their lead in the closing stages of the game as Christian Benteke fired wide after a powerful run from the halfway line, while Cabaye hit the ball wide after a Zaha effort had been blocked.
In the final minute of added-on time, Watford's Niang fired over after a goalmouth scramble but the Hornets did not deserve to take anything out of the game and Palace now sit just three points behind them in the standings.