The Hornets were without several key players and the Blades were able to establish their dominance, going in 3-0 up at the break and adding two more goals in the second half.
Jevan Beattie – son of former Watford striker Craig – was the star of the show, his hat-trick added to by Frankie Jones and Marshall Anderson.
The young Hornets did not give up, though, Max Smith hitting the bar late on – and the experience of playing at Vicarage Road should do them plenty of good.
The Blades were on top from the outset, with Beattie setting the tone upfront.
The striker scored the opener from a tight angle after just six minutes, slamming beyond Sam Morris.
Beattie and partner Marshall Francis were proving quite a threat – with both going close from inside the box.
On the half hour mark, Beattie did have his second. Alfie Atherton darted down the right-hand side and whipped a cross towards the striker, who diverted a firm header into the net.
Watford then had their best spell of the game, Kymani Daley having a few efforts on goal and Tobi Akinyimika seeing the Hornets’ best chance saved by Luke Faxon.
Just as it looked like the opening period was going to end positively, though, Blades full-back Jones curled in a beauty with his right foot to send his side in three goals up.
A similar pattern was soon established during the second half.
Watford’s claims for a foul were ignored as Sheffield United won possession, Francis eventually played in and slotting into the corner.
On the hour mark, Beattie completed his hat-trick, dispatching a penalty that he had won as he chased another ball down the channel.
Watford continued to plug away and Max Smith saw a late effort saved onto the bar, but the Blades saw the game out.
Watford: Samuel Morris, Oliver Stephenson (Tomiwa Akinyimika 64), Conrad Hunt, Jonny Scriven, Raphael London, Thomas Georgiou, Kymani Daley (Albert Stephens 64), Kiefer Gorman (Marley Clarke 85), Kash Odiase, Tobi Akinyimika, Max Smith
Unused subs: Chaniel Ogieva-Okunbor, Adrian Mariappa