It was a crushing win for the Hornets, with striker Troy Deeney netting a second-half brace, while Almen Abdi and Matthew Briggs also netted late goals.
Leon Best was sent off for the hapless Rovers, who are just two points clear of the relegation zone while Watford are just four points off the second automatic promotion spot.
There was another big win for fourth-placed Brighton, who hammered Blackpool 6-1 at the Amex Stadium with a brace from Ashley Barnes.
Will Buckley, Matt Upson, David Lopez and Andrea Orlandi were all also on the scoresheet, while an own goal from Dean Hammond was all Blackpool had to show for their afternoon’s work.
Crystal Palace shared a 2-2 draw with Leicester City at Selhurst Park with both sides vying for play-off places, and the Foxes grabbed a late point thanks to striker Chris Wood.
Bolton recorded a home victory that keeps them in the top six as they beat 10-man Middlesbrough 2-1, with Chris Eagles and Marvin Sordell on target for the hosts.
Nottingham Forest failed to make up ground on the sides above them as they remain in eighth after a goalless draw at home to relegation-threatened Barnsley, while Charlton dented Wolves’ chances of staying up with a 2-1 win over the Midlands club at the Valley.
Birmingham moved up to 10th with a hard-fought 1-0 win over Leeds, while Derby ensured that Peterborough could not move further away from the relegation zone with a 3-1 win at Pride Park.
Sheffield Wednesday grabbed an important point at home to Ipswich as they edged further clear of the bottom three with a 1-1 draw at Hillsborough.