The promotion-chasing Hornets were on track to make it five home wins from six games when Cristian Battocchio dinked the ball over Matthew Gilks midway through the first half.
However, Zola's side were unable to push on after the break and the visitors equalised when Paul Ince's son Tom flicked home a low Stephen Crainey cross.
The outcome might have been different had referee Andrew Madley awarded Watford a penalty at the end of each half after both Troy Deeney and Lloyd Doyley were fouled in the area.
Zola said: "It was one of those days that has been made like it is by one man on the pitch - and he wasn't wearing a yellow or an orange shirt.
"I would love to spend more of my time talking about somebody else and I don't like talking about the referees.
"But he's made a difference and I feel so upset, not because of him but because of my players, because they didn't deserve to lose a game like this.
"They worked very hard and they deserved something more on the pitch. The reason they didn't get it was because of him.
"He had a very poor day and he had two opportunities. He had the opportunity towards the end to at least make up a little bit for his bad day and he didn't do that and I feel furious about it."
- Sports & Recreation