Rhodes bagged his ninth goal of the season as Blackburn won to make it two consecutive Sky Bet Championship wins while also ending Watford's three-match winning streak.
The Scotland international pounced midway through the second half with a powerful header from a long throw that earned Rovers their fourth win in their last seven league outings, a run in which they have tasted defeat just once.
Gary Bowyer was sufficiently pleased with the comprehensive win at Bournemouth that he kept the same side, while Gianfranco Zola changed half of his team, despite the victory against Wigan.
By the start both sides made, the confidence coursing through the collective veins was clear, and Rhodes provided a collector's item after 15 minutes when he dragged wide from 12 yards after a neat Tom Cairney pass.
Moments later Watford showed their own intent when Javier Acuna set up the onrushing Lewis McGugan who lashed wide ferociously.
The slow reactions of Blackburn's defence almost brought an opener after 24 minutes when Davide Faraoni crossed to the completely unmarked Acuna but his header bounced off Todd Kane.
Corry Evans almost netted with only four minutes of the half remaining but his improvised volley sailed inches wide.
Watford went into the game as the league's most prolific goalscoring side with 20 and set about trying to add to that early in the second.
Troy Deeney did well to control and cross on the right byline and he found Ikechi Anya but the Scotland international thundered his volley into the ground and over.
It took 57 minutes before the first shot on target arrived but McGugan's speculative effort on his 150th career league start was easily gathered by Jake Kean.
Blackburn registered their first effort on target a minute later through Rhodes who saw his snapshot plucked out of the air by Manuel Almunia.
Almunia was again called into action moments later when Josh King slalomed his way down the left wing and into the area before undoing his good work by curling tamely at the Spaniard.
Blackburn's pressure eventually paid off and through the most simple of moves. Tommy Spurr's long throw was flicked on by Scott Dann and Rhodes was on hand to power the header past Almunia.
Watford refused to lie down, with Anya firing a long-range effort with 20 minutes left that flew wide and had Kean scrambling across his goal.
The Hornets were looking to maintain their four game unbeaten run and Fernando Forestieri combined well with Faraoni after 80 minutes but drifted too far right before firing wide.
A nerve-shredding finish was almost eased by Rhodes in injury time but Almunia saved when one-on-one, but Rovers held on, repeating the exact same outcome as last season's corresponding fixture.