Deulofeu put the Hornets in front on 18 minutes before two goals in the space of as many second-half minutes ended any hope of a fightback from the hosts, who played a role in their own downfall with some woeful defending.
The Spaniard opened the scoring after Bruno Manga gave the ball away, and doubled the advantage after a quick counter-attack. He completed his hat-trick after Harry Arter surrendered the ball tamely.
Deulofeu then set up Troy Deeney for a tap-in as Watford took a 4-0 lead.
Cardiff arguably could have done better for the visitors’ opening three goals, though they were aggrieved at 1-0 that they were not awarded what looked a clear penalty when Daryl Janmaat brought down Josh Murphy.
They did manage a consolation when Sol Bamba bundled home, but the four-goal margin was restored when Deeney scored his second from Will Hughes’ cross in injury time.
Victory sends Watford up to seventh while Cardiff remain 17th, though they could return to the relegation zone over the weekend.