The Switzerland international cancelled out Will Hughes’ second-minute opener just before half-time to clinch a 1-1 draw between the sides occupying 19th and 20th positions in the table before kick-off.
That handed the Hornets a first point of the season after three successive defeats, but represented a disappointment for Steve Bruce’s side, who had gone into the game with high hopes of building upon last weekend’s fine win at Tottenham.
Newcastle warmed to their task after a drab first half in front of a crowd of 44,157, but their poor start and a lack of precision when they did finally get going proved costly.
Bruce had suggested in the run-up to the game that six points from the first four fixtures would represent a satisfactory start to the season, but the task of collecting the three required to reach that total became significantly more difficult within two minutes of kick-off.
Tom Cleverley picked up possession 30 yards out and was allowed to advance before unleashing a shot which ricocheted off both Paul Dummett and Jamaal Lascelles and fell into the path of the unmarked Hughes, who calmly side-footed past keeper Martin Dubravka.
The Magpies should have been level within two minutes when Christian Atsu’s cross evaded all the green shirts and reached Miguel Almiron in front of goal, but the Paraguay international miscontrolled and allowed keeper Ben Foster to intervene as his wait for a first goal for the club continued.
Watford passed up two good chances to extend their lead with Kiko Femenia and Andre Gray both missing the target from promising positions as the Magpies attempted, and largely failed, to cope with the movement of Roberto Pereyra, Hughes and Cleverley’s invention.
Jetro Willems saw an ambitious volley from Atsu’s 26th-minute corner blocked at source with Bruce’s men mounting a somewhat tepid response, and Almiron found the back of the net after rounding Foster 11 minutes later, but the offside flag had gone up long before he did so.
The home side returned after the break in positive mood and Schar might have doubled his tally from Sean Longstaff’s 55th-minute free-kick, but headed wide before Almiron saw his shot from Atsu’s pull-back blocked by Christian Kabasele.
Foster managed to tip Hayden’s 59th-minute piledriver over, although with some difficulty, although he dealt with Sean Longstaff’s skidding effort far more comfortably four minutes later.
Dubravka had to save twice from substitute Isaac Success as the visitors mounted a late charge which also saw Pereyra deflect a Cleverley shot inches wide, but it was Yoshinori Muto who might have won it for the Magpies at the death, only to shoot into the side-netting.