The second-placed Hornets looked on course for a seventh straight win when Ismaila Sarr's fortuitous goal in the 32nd minute stood despite Boro's claims for handball and offside.
But ninth-placed Boro, who are now six points off sixth spot with six games remaining in the Sky Bet Championship, found an extra edge in the second half of the lunch-time kick-off.
And substitute Bolasie, on loan from Everton, headed in the equaliser with 12 minutes remaining to prevent Watford from making the five-point gap to leaders Norwich even smaller.
Boro initially struggled to hold onto possession and the visitors dictated a lot of the early play without creating many chances.
The nearest Watford came to scoring before the breakthrough was from a Philip Zinckernagel effort that deflected for a corner.
With hardly any time to settle into a new approach, as George Saville was asked to drop deeper, Boro fell behind.
When Marc Bola cleared straight to a Watford shirt, Zinckernagel appeared to control with the help of his arm before a goalbound effort deflected off Sarr's heel and beat a wrong-footed Bettinelli.
Boro finished the half stronger but the lack of an out and out striker in the starting line-up left them struggling to get players in the box to get on the end of crosses.
There was a goal ruled out early in the second half when Saville's header was turned in by Duncan Watmore, who was a yard offside in the six-yard area.
Watford failed to find full flow but with their first bit of quality in the second half Joao Pedro side-footed wide of the near post after Sarr's fine run and cut back.
Pedro ought to have done better when he was picked out unmarked by Sema, but the shot curled wide of the far post.
But Boro found a new lease of life after the introduction of substitutes Bolasie and Chuba Akpom and in the 78th minute they levelled.
Boro finished strongly without testing Bachmann again and Watford edged a little closer to a return to the Premier League with a point.