Neil Warnock's side could have moved up to 13th with a win – four points ahead of 18th-placed Southampton – but was undone by a trio of superb finishes from Watford's Spanish forward and two more from Troy Deeney.
The Bluebirds – who scored a late consolation through Sol Bamba – were aggrieved not to be awarded a penalty at 1-0 down, but they can scarcely have any complaints with the result after barely laying a glove on the Hornets over the 90 minutes.
The defeat brings a run of two consecutive wins to an end and means they could end the weekend in the drop zone if the Saints get a point against Arsenal on Sunday, while Javi Gracia's side temporarily leapfrog Wolves into seventh.
Just two minutes after Ben Foster thwarted an instinctive Oumar Niasse effort, Watford forged ahead.
Deeney showed superb persistence to retain the ball on the edge of the penalty area before teeing up Deulofeu, who coolly slotted into Neil Etheridge's bottom right-hand corner from 20 yards.
The Cardiff goalkeeper got down well to palm away Deeney's powerful drive before the half-hour mark, while the hosts were denied what looked like a certain penalty when Janmaat brought down Murphy in the area shortly before the interval.
Any hopes Cardiff had of getting back into the game were extinguished just after the hour as Deulofeu scored twice in two minutes.
His first came after a surging run from his own half, rounding Etheridge to coolly slot home, and he completed his hat-trick with a sumptuous chip over the Cardiff goalkeeper after Harry Arter had cheaply given the ball away in his own half.
The former Barcelona man then turned provider for Watford's fourth after 73 minutes, teeing up Deeney to slam home from 10 yards before Bamba bundled home from close range eight minutes from time.
Gracia's side had the last say, though, Deeney slotting home from Will Hughes' pull-back in stoppage time to seal a memorable away win.