Darius Henderson found the side-netting after rounding Andy Lonergan and Paul McKenna hit the angle of post and bar for Preston in a bright opening.
Lonergan tipped a Jobi McAnuff drive wide, John Eustace's header was cleared off the line and Dan Shittu's header hit the post as Watford surged forward.
But Richard Lee saved well from Richard Chaplow and Chris Brown as North End earned a first draw under Alan Irvine.
Watford manager Adrian Boothroyd:
"That's the most dominant we've been for a while but if you don't take your chances then you don't win games.
"That's the first game here this year which we haven't won. If we can maintain that home form and keep picking up points away then we will be there or thereabouts at the end."
Preston boss Alan Irvine:
"We had to be resolute defensively and determined and brave - both mentally and physically - because you get put under pressure from Watford.
"In the second half, we passed it better which gave our lads at the back a bit of a breather and caused them a few problems."
Watford: Lee, Mariappa (DeMerit 81), Sadler, Shittu, Bromby, Smith (Ainsworth 85), Eustace, O'Toole, McAnuff, Henderson, Ellington.
Subs Not Used: Stewart, Williamson, Kabba.
Preston: Lonergan, Jones, Chilvers, Mawene, Davidson, Sedgwick, Chaplow (Hill 89), McKenna, Whaley (St. Ledger 84), Hawley (Mellor 72), Brown.
Subs Not Used: Chris Neal, Nicholls.
Booked: Davidson, Mellor.
Att: 16,798.
Ref: Tony Bates (Staffordshire).
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