Josh Sargent scored a double before Juraj Kucka’s late own goal, with Emmanuel Dennis sent off for the Hornets who were humbled by a fellow relegation battler.
The win means the Canaries move out of the drop zone, having been mired down the bottom all campaign, but the pressure is now mounting on ex-Leicester City title winner Ranieri, who took over at Vicarage Road in October.
“Before I answer I want to say sorry to our fans because we wanted to do something better,” Ranieri told BBC
“We asked the players to light with fire and we can’t lose this match, but after the goal we disappeared.
“Maybe we played better 10 v 11. It is very important the lesson Norwich gave us, they played as a team, we played as individuals. That is no good.
“Now I have to choose the players that want to fight hard for Watford. You have to play 11 v 11.”
Ranieri has vowed to fight on with the Hornets but their plight is difficult, having collected one point from their past eight league games.
Norwich are up to 17th after back-to-back wins, with 16 points from 22 games.