Neil Warnock's side were desperate for maximum points against relegation-threatened Watford, but despite drawing a blank Reading's defeat to Cardiff and Brighton's injury-time equaliser against West Ham keeps the Blades' faint hopes alive.
United improved drastically after a poor first half, but were unable to convert their limited chances, while Watford, still looking to ensure their Coca-Cola Championship safety, fought as their lives depended on it.
Watford, already without ineligible Blades defender Danny Cullip, were dealt another blow when midfielder Brynjar Gunnarsson was replaced before kick-off by Dominic Blizzard after sustaining an injury during the warm-up.
Jay DeMerit returned from suspension to fill in for Cullip, on loan until the end of the season as part of a swap with Danny Webber, who was also ineligible for Neil Warnock's side.
Warnock was able to recall fit-again Michael Tonge to his midfield where Paul Thirlwell was preferred to Nick Montgomery, while Steve Kabba stepped in for Webber up front.
Neither Watford, desperate for a maximum return to haul themselves clear of relegation trouble, or United, still clinging to an outside chance of making the play-offs, created a clear-cut chance in the first 25 minutes.
But then both sides went close, first DeMerit heading the prominent Chris Eagles' cross over the crossbar and Kabba seeing home goalkeeper Alec Chamberlain hold his stinging shot from the edge of the penalty area.
Derek Geary then volleyed over for United, while Watford dominated possession without carving out any openings.
Warnock sent on a striker for a defender after the interval, Jonathan Forte replacing the injured Chris Morgan and within a minute the 18-year-old almost snatched the lead for the Blades, drilling a left-foot shot from the edge of the area that had Chamberlain parrying at full stretch.
Jon Harley curled a free-kick over from 25 yards and then saw his volley from similar distance blocked by Neil Cox, while Andy Gray also blasted over as United stepped it up in a bid to keep their season alive.
Tonge forced Chamberlain into action again with a far-post header from Harley's corner, while Eagle pulled his shot wide in a rare foray forward for Watford.
Bruce Dyer then tested the ever-alert Paddy Kenny with a long-range effort and Ashley young also let fly before Cox headed over the bar as Watford hit back after riding the storm.
Warnock threw on striker David Johnson for midfielder Alan Quinn in the 79th minute, but apart from Kabba's late shot that was blocked by DeMerit the visitors struggled to make headway against stubborn opposition.