Gerrard, the outstanding player on a disappointing night, had already nosed the Premiership club in front with a goal in the first leg.
He put the matter beyond doubt when he pounced on a midfield mistake by Watford's Paul Devlin and broke forward before drilling a low shot to the right of Paul Jones.
Liverpool's concern after eight days in which they lost two Premiership matches and were knocked out of the FA Cup was clearly underlined in their approach to this match against the Championship side.
They strung five players across midfield and spent much of the night giving Watford no space and they did so very effectively.
They were so much in control that Watford hardly had a chance and Jerzy Dudek in the Reds goal did not have a save to make all evening.
The one real scare for the Oremiership side came just before half-time when Hornets' Hameur Bouazza appeared to be felled by a tackle from behind in the box by Steve Finnan.
Referee Mike Riley waved away Watford's appeals although video replays showed it should not have been lightly dismissed.
Large periods of the match were tedious though although the home fans consoled themselves with rounds of applause and cheers if their team showed some skill in winning throw-ins.
So effective was Liverpool's midfield blanket that the game rarely rose above the mundane.
Gavin Mahon's off-target shot was Watford's only real goal attempt in the first half while at the other end Fernando Morientes collected a pass from Gerrard but saw his goalbound shot deflected just wide by Watford's outstanding defender Neil Cox.
Even so Watford seemed comfortable in snuffing out most of Liverpool's attacks but ran into cul-de-sacs when they attempted to move over the halfway line.
Morientes headed well wide early in the second half but Watford caused another scare when Bouazza attempted to place a shot beyond Dudek only for Djimi Traore to intercept with a sliced clearance for a corner.