© PA Photos
Millwall had their first effort on goal through James Henry on 13 minutes, but the midfielder's header went well wide of the far post.
Deeney continued his battle with Forde soon after, but scuffed his effort from close range.
Yeates had time and space from close range but his effort lacked power and was tipped wide by a good save from Forde.
The home side improved vastly after the break but were fortunate to stay level when Deeney had a flicked effort cleared off the line.
Danny Shittu then escaped punishment after appearing to handle in the area but the Lions rallied and made the breakthrough just seven minutes from time.
Substitute Batt was on hand to tap home unmarked from close range and condemn Watford to a crushing defeat in their search for automatic promotion to the Premier League.