While Newcastle's performance had much to admire it was a result that will do nothing to ease the pressure on manager Steve Bruce whose side sit just above the relegation zone.
"In the first half alone we should have been three or four goals in front," said midfielder Sean Longstaff, who gave Newcastle a deserved 23rd-minute lead.
"We had a lot of very good chances, more than we'll create in most games, and it is very disappointing to come away with only a point. It was the best we've played this season."
Newcastle fans chanted against Bruce and owner Mike Ashley in the opening stages but they were cheering when Allan Saint-Maximin found Longstaff from a corner and the midfielder curled an unstoppable effort past Ben Foster.
Longstaff wasted a great chance to double Newcastle's lead, firing over the crossbar just before halftime after Saint-Maximin latched on to a poor back pass.
Bruce's side were unable to hold on to their lead, however, and Josh King and Sarr linked up to bring the hosts level on 72 minutes.
King's flick-on from Danny Rose's corner found Sarr in space at the back post and the Senegalese international powered a header past Darlow.
King almost grabbed a late winner but his close-range finish was ruled out for offside after a VAR check.
Despite another winless weekend, results elsewhere meant that Newcastle edged up to 17th in the table with three points, with Sarr's rescue act keeping Watford in 11th with seven points.
The game was noteworthy for the fact that the referee was Australian Jarred Gillett -- the first overseas referee to take charge of a Premier League fixture.