Cliff Byrne and Martyn Woolford scored either side of half-time to lift the Iron into 14th place as rookie boss Ian Baraclough followed up his first win as manager at Preston in midweek with another victory.
Woolford had the first shot in anger for the visitors in a bright start but it was well blocked before Michael O'Connor's corner was well met by David Mirfin but his header was easily held by Scott Loach.
The visitors continued to impress and Loach had to be at his best minutes later when he got down sharply to parry Chris Dagnall's shot away.
And the Iron made the most of their bright start when Byrne broke the deadlock in the 28th minute.
Another O'Connor set-piece was only cleared as far as the edge of the area and Byrne drilled in a low strike which arrowed into the bottom corner.
The hosts responded well and youngster Michael Bryan saw a stinging long-range drive turned over by debutant goalkeeper Tony Warner as the Hornets looked for a response before half-time.
There was a second goal before the break but it came from the away side through the impressive Woolford.
Byrne crossed from the left in first-half stoppage time and when the ball was not hacked clear, Woolford was on hand to swing his left boot and send the ball arcing past Loach and into the far corner.
Watford strikers Marvin Sordell and Danny Graham had been quiet in the first period but combined at the start of the second period when Graham got a shot in which Warner was right behind.
Sordell then climbed well to flick a header wide of the near post before Freddie Sears forced a fine save from Loach at the other end after a fine solo run from inside his own half.
Stephen McGinn and Jordon Mutch both went close to pulling a goal back for the hosts before centre-half Adrian Mariappa had the best chance in the dying moments but saw his header clear the crossbar.