© PA Photos
It looked like neither side would break the deadlock before the break, but Deeney picked up the ball in the 41st minute to give the visitors the lead, firing home from just outside the penalty box.
Jonathan Bond was called into action early in the second half as Hull pushed for an equaliser, but the Watford keeper was able to block Jay Simpson's shot, before collecting an 18-yard effort from Robbie Brady.
Deeney almost scored his second of the game just after the hour mark, but a one-handed save from David Stockdale ensured that the visitors' advantage remained at one goal.
The home side kept Bond busy as they looked to get back on level terms, forcing a save from the keeper after Robert Koren mishit a shot from 20 yards, which was followed by two more efforts from Brady.
Despite having the majority of the chances in the second half, Hull were unable to find the equaliser before the end of the full-time whistle.
The result leaves Watford just one point behind Hull in the Championship table.