Hull took the lead within 47 seconds of the kick-off through Michael Turner, before goals from Fraizer Campbell and Caleb Folan sealed the victory for Phil Brown's men.
Hull took the lead within seconds of the kick-off as Dean Marney floated in a right-footed free-kick from just outside the area which found the head of Turner, who sent the ball into the bottom corner of the net with a neat header from six yards.
The home side were then 2-0 up on thirteen minutes as the ball fell to Henrik Pedersen in the box.
He fired in a first-time volley which Campbell managed to turn past keeper Richard Lee and into the back of the net from five yards to register his 14th goal of season.
Jobi McAnuff came close to grabbing one back for Watford towards the end of the half, but his arrowed right-footed shot from 20 yards was well parried away by Hull keeper Boaz Myhill.
Watford nearly pulled one back on the hour-mark as the ball fell to Lee Williamson in the box, who turned and struck a shot from close range, but keeper Myhill showed great bravery as he rushed out to parry the ball away.
Substitute Folan then sent Hull into a 3-0 lead as he chased down on a through ball and slipped the ball underneath the oncoming Lee and into the back of the net from eight yards.
Watford were then down to ten men on 74 minutes as Steve Kabba was adjudged to have jumped in dangerously on Sam Ricketts and was sent off by referee Uriah Rennie.