Yannick Bolasie rescued a point for Boro on Watford's last visit to the Riverside.
The Hornets were Premier League bound, and arrived on Teesside hoping to add to a run of sixth straight wins.
Neil Warnock's Boro, on the other hand, had won the previous two at home before defeats on the road at Millwall and Bournemouth had dented play-off ambitions.
Watford took a first-half lead in the behind-closed-doors contest as Philip Zinckernagel's shot hit Ismaila Sarr and found the back of the net.
But substitute Bolasie earned Boro a share of the spoils when he headed home from Paddy McNair's free-kick.