Scoring a goal either side of half time, it was never too much of a struggle for Liverpool, very much still on a high after Thursday's Europa League semi-final against Villarreal.
Eight changes, three points
Jürgen Klopp made a number of changes, as was expected, with the likes of Christian Benteke, Martin Skrtel and Sheyi Ojo all lining up in red.
One surprise was the selection of Simon Mignolet in goal, with Danny Ward expected to be given another opportunity to impress, however it was later revealed that the Welshman was struck down by a knock picked up in training.
Both teams on the offensive in entertaining first half
Those changes didn't seem to have much effect on the performance of Klopp's team, who got at the Hornets early on in the Merseyside sunshine.
Philippe Coutinho nearly opened the scoring with a deflected effort shortly after the 10 minute mark, shooting from the edge after getting the better of Craig Cathcart. Ojo was too looking dangerous, having eased his way into the first team squad over recent months.
Watford weren't going to roll over without a fight though, and did trouble the Liverpool goal on more than one occasion in the first half. Ikechi Anya wasted a chance after being set up by Odion Ighalo, striking over the bar from 18 yards out. Almen Abdi, too, wasn't far away with Skrtel blocking his goalbound shot.
Allen nets the opener
The opener did go to the hosts though, Joe Allen making it 1-0 just short of the interval. Christian Benteke used his strength and power to knock the ball down to his teammate in the area, Allen nipping in to divert the ball past Heurelho Gomes and into the net.
That goal forced Watford, who had initially looked to counter-attack, further forward in the second 45, presenting Liverpool with more room when breaking. Coutinho should have done better not long after the restart, dragging an effort wide after good work from Kevin Stewart and Benteke in the build up.
Mignolet's inclusion ahead of Ward's fellow reserve 'keeper, Adam Bogdan, was certainly justified as the Belgian saved brilliantly from Watford's best chance of the game. Anya's defence-splitting pass found Ighalo, who jinked past Alberto Moreno before curling an effort towards goal from inside the area. With the Watford bench half up celebrating, an oustretched hand from Mignolet somehow managed to tip the ball away.
Firmino on to seal the points
Roberto Firmino's introduction gave Liverpool further control and threat going forward, the Brazilian almost scoring with his first touch as Gomes was forced into a good save. The 'keeper couldn't keep him out for a second time though, Firmino sealing the win with 15 minutes to go.
Ojo pressed the ball well, taking it from Anya to set up the Brazilian, whose shot found it's way into the bottom left hand corner after Gomes failed to get a strong enough hand on it.
Liverpool had chances to make it three, Ibe hitting the post after Firmino gave him an opportunity that looked harder to miss at the back post, before Benteke headed over from Cameron Brannagan's cross in stoppage time.
Still, three welcome points for the Reds, who take momentum into the midweek clash against Chelsea.