In a predictably tight contest between two of the three relegated sides, the Hornets took a grip on the game at the start of the second period and deserved to make the breakthrough.
Yet the opening 45 minutes were curious because there was a feeling that the team which took the lead would go on and win comfortably, but neither side had sufficient control of the game to suggest they would be the ones to take the lead.
After the Blades had shaded the opening exchanges and the unmarked Leigh Bromby had headed straight at Mart Poom following a corner, Watford enjoyed the best spell of sustained attacking pressure in the first half, but neither Tommy Smith or Williamson were able to trouble Paddy Kenny with headers.
But the Hornets upped the ante after the break and deservedly went in front after 56 minutes.
A good ball from Smith found Darius Henderson on the left and, after weighing up his options, the striker superbly cut the ball back to the edge of the area where Williamson met it with a first-time clipped half-volley that beat the diving Kenny at his near post.
Having set up one goal, Henderson so nearly made it two minutes later when he powered a close-range header narrowly wide from a corner and then Marlon King unleashed a trademark long-range effort that wasn't too far away either.
But Watford really should have made the game safe in the 70th minute when Henderson got goal-side of a defender to latch onto Jobi McAnuff's pass inside and closed in on goal, but he placed his shot too close to Paddy Kenny.
And, had it not been for a fantastic intervention from Smith moments later, the Hornets would have been made to rue that miss.
The Watford winger appeared from nowhere to clear the ball from inside the near post after Poom had only been able to get a hand to a Chris Armstrong free-kick that went under the wall.