Watford, who are now four points clear of the relegation zone, took the lead in spectacular fashion when the impressive John Eustace hit a sumptuous volley from the edge of the penalty area.
Despite long periods on the back foot, Boro were a constant threat on the break and Lita struck nine minutes into the second half, shortly after entering the fray.
Middlesbrough goalkeeper Brad Jones enjoyed some early action when he athletically tipped Tom Cleverley's 20-yard free-kick around the post after Stephen McManus had fouled Heidar Helguson.
The Hornet's next opportunity came from another Cleverley free-kick, as Martin Taylor failed to make clean contact with a dangerous cross when well placed.
In the 18th minute Middlesbrough's Barry Robson got behind the home defence to fizz a dangerous low ball across goal which Henri Lansbury scrambled clear.
Watford took the lead two minutes later when Don Cowie's corner was only partially cleared by Chris Killen and Eustace struck a controlled 20-yard volley past the helpless Jones for his fourth goal of the season.
The home side were now in the ascendancy, with the livewire runs of Will Buckley causing Middlesbrough particular problems.
Seven minutes before half-time, Helguson flicked on Eustace's ball forward for Buckley to shoot into the side netting with his weaker right-foot from 12 yards.
Watford continued to press and three minutes after the interval and Helguson laid off a Don Cowie cross for Cleverley, but the midfielder could not direct his shot goalwards.
Strachan introduced Lita minutes into the second half and in the 54th minute he brought Boro level.
Gary O'Neill overhit his cross following a swift counter attack, but Julio Arca retrieved the ball on the left byline and Loach could only palm his lob across goal before Lita beat Lloyd Doyley to the loose ball from close range.
Arca began to orchestrate matters in midfield as Boro gained the upper hand, but Helguson fashioned space for himself in the 58th minute before smashing over to show Watford retained their ambition.
In the 65th minute Cleverley tried his hand once more and Jones was relieved to see his effort travel past the upright after a wicked deflection off Justin Hoyte left him stranded.
Taylor was then on hand to block a Lita effort after another clinical Middlesbrough break.
The former Reading man went on a rampaging solo run before firing narrowly wide via an Adrian Mariappa deflection from 12 yards in the 75th minute.
Moments later, Jones punched tamely clear and Cleverly lobbed narrowly wide of the unguarded Middlesbrough net.
Helguson had a golden chance to snatch the points for Watford in the 83rd minute but he prodded wide at the back post after Cleverley's arching cross evaded the Boro backline.