Sloppy marking by Forest the defence - the biggest factor in their poor league form this season - gifted the away side all three points and condemned Forest to their third home defeat of the campaign.
The Reds were on the receiving end of the same treatment by Preston North End in their previous league outing a week ago when they conceded three first-half headed goals.
There was little on show to suggest that Joe Kinnear has taught his players how to man mark as both Helguson goals could have been prevented had there been a red shirt somewhere near.
The routine for both away goals involved Neal Ardley who produced pin-point crosses to the goalscorer - the first coming 20 minutes after the start of the game and the second seven minutes later.
An rare headed goal by Andy Reid on the stroke of half-time from an unlikely cross from central defender Michael Dawson produced a lifeline for the home side who were booed off the pitch at the interval.
It was all set for a home fightback in the second half but the ineffective strike partnership of David Johnson and Marlon King gave little hope that anything could be salvaged.
Only Reid looked likely to find the equaliser but with little support from his team-mates the deficit remained.
Watford goalkeeper Richard Lee produced a breathtaking save from a Wes Morgan header in the dying seconds to keep his side ahead.