Heidar Helguson's 50th-minute goal, his tenth of the campaign for the Hornets and 11th in total, made the difference as Malky Mackay's men moved three points clear of the drop zone and with a game in hand on most of their rivals at the wrong end of the table.
The on-loan Queens Park Rangers striker was in the right place to tap home after John Eustace beat keeper David Stockdale to a Don Cowie cross and, although the header failed to have sufficient power to drop over the line, Richard Eckersley failed to clear.
The Iceland international should have opened the scoring moments earlier when a Cowie free-kick dropped invitingly to him six yards out, but he scooped the ball over the bar.
Neither keeper had been troubled during a scrappy first half, but it was the Hornets who had the better of it as the opening period wore on and with a bit more good fortune they would have been in front at the interval.
First, Danny Graham scuffed a shot into the ground, but the ball looped beyond Stockdale, only to rebound to safety off the woodwork.
Then a quick throw-in from Graham released Helguson in the area and he sought to pick out Cowie on the edge of the six-yard box, but he was beaten to the ball by a defender, who diverted the ball narrowly wide of his own far post.
Plymouth sought to get back on terms after falling behind but, despite huffing and puffing and being aided by the home side's tendency to defend too deep, they struggled to create anything of note and did not test Watford keeper Scott Loach in the entire contest.