But Gomes was at fault before the visitors passed up a glorious opportunity to take the lead with a little over 10 minutes gone.
For Swansea, though, the gloom deepens as they remain entrenched in the bottom three - the bright start to Paul Clement's tenure a distant memory - two points shy of fourth from bottom Hull.
Etienne Capoue capitalised on Mawson's sloppiness to score the only goal of a tiresome encounter, the 1-0 win taking Watford onto 40 points and all-but securing their top-flight status.
"It was only West Ham where anxiety was there for everyone to see", he said.
Their sole consolation from another tough away afternoon came in the form of 17th-placed Hull City also losing, beaten 3-1 at Stoke City.
Fernandez fouled M'Baye Niang on the edge of the Swansea box but the AC Milan loanee could only drill the resulting free-kick straight at Lukasz Fabianski.
That will not be good enough for Swansea, but a point would suit Walter Mazzarri's Watford absolutely fine. He caught Mawson napping in possession, robbing the Swansea defender before turning home at the second attempt.More news: Donald Trump offers trade deal with China to confront North Korea threat
Early in the second half, the lively Sigurdsson fizzed a 20-yard effort narrowly wide of Gomes' left post, before Watford responded through Daryl Janmaat, whose long-range effort forced Fabianski into a smart stop.
- Watford have kept three consecutive clean sheets on home soil in the Premier League for the first time since September 2015.
- The Swans have failed to score in four of their last five Premier League games (one goal scored in the other), after finding the back of the net in seven in a row.
The 32 year-old Spaniard has 11 goals in 27 appearances for the Swans this season, and it could be his goals that keep Swansea in the Premier League come the end of the season - if he's fit its imperative he starts.
Swansea City boss Paul Clement is urging his players to use Cristiano Ronaldo as inspiration.
Sigurdsson and leading scorer Fernando Llorente, restored to the starting line-up by Clement after recovering from injury, both had shots on target as Swansea turned the pressure on Watford but Gomes answered the call on both occasions.