A vintage Peter Hutton goal on 75 minutes earned Derry City all three points at the RSC and extended Waterford United's losing streak to five games.
Ruaidhri Higgins provided the cross from the left wing and the Blues defence, having defended valiantly all night, displayed yet another inexplicable lapse of concentration as they failed to clear the danger.
Kevin McHugh almost put Derry in front in the 62nd minute when, after winning possession in midfield, he spotted the Blues goalkeeper off his line. The substitute's lob was inch perfect but Dean Delaney back-tracked furiously to tip the ball out from under the bar, injuring his back in the process.
United will be disappointed not to have taken a point, at least, from a game they dominated for long periods.
The Blues had the best of the first half but, despite subjecting their visitors to almost unrelenting pressure, they were unable to open their account.
Paul McCarthy smacked his looping shot on the run just wide of the upright on nine minutes and, on 23 minutes Vinnie O'Sullivan headed the ball into the arms of goalkeeper Pat Jennings following a long throw-in from Alan Reynolds that was flicked on by David Breen.
A minute later, a free-kick by Reynolds was only partially cleared by Sean Hargan and Jennings had to dive full-length to turn a rasping, low shot by Sullivan around the post for a corner.
On 38 minutes, a corner from the right by Dave Warren was headed out of a crowded Derry goalmouth and Reynolds almost conjured up a goal but the ball shaved the outside of the upright as he drilled it back through a forest of legs.
Derry only managed two half chances in the first period, a snap-shot from Peter Hynes on 9 minutes that went wide and a cross by Higgins on the half-hour that skidded across the face of the United goal but there was nobody on hand to apply the final touch.
Waterford United: Delaney, Scully, Flynn, Keely, Breen, Hayes, Reynolds (Mulcahy, 83 minutes), McCarthy, Sullivan (Kavanagh, 66 minutes), Warren (Harte, 77 minutes), Brosnan. Substitutes not used: Cooling, Holden.
Derry City: Jennings, Hargan, Hutton, Rogers, Higgins, Brennan, Oman, McGlynn (Moore, 66 minutes), Farren, O'Halloran (Deery, 85 minutes), Hynes (McHugh, 56 minutes). Substitutes not used: Tidman, Molloy.
Referee: Alan Kelly
