With half-time looming, the centre-back had the ball on the edge of the Swansea box but allowed the pressing Etienne Capoue to get just a little too close, dallied on the ball and was tackled, leaving Capoue with only Lukasz Fabianski to beat.
The Frenchman’s first effort caught the keeper’s toes but, as it bounced back, he rifled a left-footed half-volley into the net.
After their narrow (almost identical) defeat to West Ham last weekend, and their late collapse against Tottenham before that, this result will only add to the pressure ahead of a crucial game with Stoke at home next week.
Boss Paul Clement said: “Hull not winning keeps things the same but for me it’s a missed opportunity not to get a point today.
“The next game is a must-win game for us. It’s the biggest game the club has had in years. We will probably need three wins out of the last five to stay up. And I’m not even sure that would be enough.”
This was a game that mattered more to the visitors and Swansea began it with the intensity to match.
But it was a game of hard work but very few sights of goal.
Swansea’s talisman and captain Gylfi Sigurdsson went close in the ninth minute, when he was first to reach a Luciano Narsingh cross from the right, and, after his first shot was blocked well by Adrian Mariappa, he forced Heurelho Gomes into a smart parry to his left.
And Sigurdsson demonstrated his confidence when he had a go from over 35 yards out, forcing Gomes into an agile stop with a swerving half-volley in the 32nd minute.
But that was about it. The result took Watford to the magical 40-point mark.
But Hornets boss Walter Mazzarri said: “Until the numbers say we are safe, we are not. I don’t want my players to relax.”
