The result leaves the Bluebirds five points clear at the top of the Championship, while the hosts are three points behind Hull City.
Malky Mackay's side began brightly, focusing their attacks mainly down the right-hand side.
Watford then come into the game and had a few opportunities of their own.
Then Craig Conway made a quick breakaway, but dragged his shot wide from just outside the box.
With plenty at stake for both sides, the play became heated as the first half drew to a close, leading to a few crossed words between the dugouts.
Cardiff had Marshall to thank for keeping the scores level in the 55th minute, as he sprang to his right to tip Deeney's header around the post.
That was the only real chance in a second half that was keenly contested but very tight.