Matthew Spring's first-half double set up Watford's win, the midfielder powering in a shot on 18 minutes and then applying a close-range finish.
Marlon King made it 3-0 just before the hour when the Watford striker drove home from the edge of the penalty area.
Three minutes later Ashley Young added his name to the scoresheet, firing home from close range, before Luke Rodgers volleyed home for Crewe on 81 minutes.
Crewe manager Dario Gradi:
"We've been in this situation before and there are no magic words.
"We will look at the video, pick up on the mistakes and we'll go out and work to try and eliminate them.
"I don't have to pick the players up, they pick themselves up. They have a good spirit and attitude to everything they do."
Watford: Chamberlain, Doyley, Mackay, DeMerit, Stewart, Eagles (Chambers 74), Mahon, Spring, Young, Henderson (Bouazza 54), King (Bangura 65).
Subs Not Used: Carlisle, McNamee.
Goals: Spring 18, 26, King 58, Young 62.
Crewe: Turnbull (Tomlinson 45), Moss, Foster, Walker, Otsemobor, Gary Roberts (Rodgers 79), Vaughan, Lunt, Billy Jones, Taylor (Varney 77), Steve Jones.Subs Not Used: Cochrane, McCready.
Goals: Rodgers 81.
Ref: K Friend (Leicestershire).