Algerian Hameur Bouazza scored for Watford.
"The quality of our game when we got possesion was fantastic," Ferguson said.
"Rooney was out of this world."
United will play either Chelsea or Blackburn, who play Sunday at Old Trafford, in the final on May 19 at the new Wembley Stadium.
Rooney gave United the lead in the seventh minute with a powerful shot
from the edge area after receiving a pass from Michael Carrick.
Bouazza then equalized in the 26th with a superb overhead volley from close range, but Ronaldo scored his 21st goal of the season in the 28th, a day after signing a five-year contract extension with United.
Rooney cut in from the right and passed to Ronaldo, who nudged the ball
into the net.
Despite a shaky start to the second half, Rooney scored his second goal in the 66th and Richardson, who came on for Ronaldo, scored in the 82nd.
Watford, which is in last place in the Premier League, struggled to break
down United's dominance. But Bouazza scored his team's lone goal shortly after a throw-in when his shot deflected off United defender Gabriel Heinze and into the net off the crossbar.
"They were just too strong for us and we didn't take our chances when we had that great little period in the second half," Watford manager Aidy
"It wasn't as one-sided as the scoreline suggests."
United leads the Premier League and has also reached the semifinals of the Champions League, where it will play AC Milan.
The FA Cup final will be the first major match to be played at Wembley.
The stadium reopened last month after more than six years under construction.
United hasn't lost in the FA Cup semifinals since 1970 and started
Saturday's match with the high tempo game plan that routed AS Roma 7-1 Tuesday in the Champions League.
After Rooney's early goal, Alan Smith had a chance in the 12th but he sent the ball over the bar.
Watford defender Jay DeMerit made a stop on Paul Scholes in the 21st, and, after Ronaldo restored United's lead, the United States defender blocked the Portugal winger in the 30th.
Rio Ferdinand limped off in the 40th to join Nemanja Vidic, Gary Neville
and Mikael Silvestre on United's injury list in defense.
Substitute Darren Fletcher dropped to right back, while Gabriel Heinze
moved to the center to partner with Wes Brown.
The treble hopes of United may rest on Ferdinand’s recovery.