With his side 20-12 ahead, the controversial prop produced arguably his daftest dismissal yet with a bizarre headbutt on Dom Crosby in the 72nd minute.
Watts – sent off three times last season – was walking forward as if to play the ball but then, strangely, lowered his head at the Warrington player leaving referee Ben Thaler with no option.
Hull chief Lee Radford said: “It was pathetic. A pathetic attempt at a headbutt first and foremost, and a crazy time to do it.
“It’s a crazy time to do anything like that, obviously, in the game.
“He’ll get a suspension and he’ll probably deserve it.
“I was really pleased when we went 18-0 up, but really disappointed with everything after that.
“Sometimes when you go a man down you can get galvanized and try and push things and they looked dangerous when they threw it about.
“I was panicking towards that back end. I thought we almost committed suicide but it was nice to get that win.”
Earlier, the official had already brandished a red card to Warrington’s Declan Patton for a dangerous swinging arm on Bureta Faraimo in the 48th minute.
Patton, in the Wire side given England stand-off Kevin Brown was still suffering from concussion, ended up giving Faraimo a headache of his own.
The winger had broken clear of a few tacklers when Patton came in from his blindside with a swinging arm from behind that caught the New Zealander clean on the jaw.
With Faraimo needing urgent treatment – he was also knocked out in the second minute of Hull’s game against Wigan in Wollongong only last month – Thaler consulted with his colleagues before reaching for the red card.
That should have seen the hosts ease home on a freezing night at the KCOM Stadium; Sneyd wonderfully curled over the resultant penalty, into the wind and snow from 40m wide out, to extend their lead to 12-0.
Then, soon after, the scrum-half crabbed across field and picked out winger Fetuli Talanoa cutting back on a perfect line to dissect Warrington’s defence, Carlos Tuimavave providing the supporting finish and Sneyd making it 18-0.
However, 12-man Warrington were roused into action and turned the game on its head with two tries in quick succession.
With their first real chance since the early exchanges, ex-Huddersfield Giants second-row Jack Hughes crossed off Stefan Ratchford’s short pass in the 56th minute and they did so again in the next set when veteran Ben Westwood drove close and squeezed out an offload for Ryan Atkins.
Shaky Hull had to desperately scramble to avoid conceding a third successive score when 36-year-old former England back-row Westwood found another offload to set Daryl Clark free.
Radford’s side survived, though, and Sneyd’s penalty after Tom Lineham’s high tackle on the excellent Jamie Shaul in the 71st minute seemed to have alleviated all the pressure – until Watts’s moment of madness.
Nevertheless, Sneyd made sure with a drop goal in the final seconds but it was far from the performance Radford will have hoped for.
That said, it was a first win in four games, only a second of the campaign and much-needed.
In his programme notes, Hull chairman Adam Pearson detailed how his side’s results this month would decide whether their recent trip to New South Wales –where they lost to Wigan Warriors and St George-Illawarra – had been beneficial.
To that end, they started March with a win and without injured stars Albert Kelly and Danny Houghton too, but they will need to improve further at champions Leeds Rhinos next Thursday.
Pearson wrote: “Our aim for March, like the other clubs, will be to prove the tour was worthwhile and will not have a detrimental effect on our season, which will inevitably have a bearing on whether clubs opt for similar experiences in the future as the global reach of our sport continues to be widely discussed.”
Hull, with Jake Connor switching to six in place of Kelly, were good value for their 10-0 interval lead by sheer virtue of making few errors and being better disciplined than their rivals, too.
Warrington were on the end of a 6-1 penalty count at the half-point but they could have few complaints, Ben Murdoch-Masila being particularly soft with his needless interjections.
Similarly, the kicking of Sneyd and Connor, even when looking relatively harmless, caused the visitors’ wide defence no end of problems.
The only try of the first period came after Connor’s astute 40/20 kick. In the resulting set, after the returning Sika Manu was held up, Toby King flapped at a Sneyd chip to let Manu pick up the pieces and loft a pass out to unmarked winger Faraimo.
Sneyd converted having earlier slotted a penalty, King – on for the injured Bryson Goodwin after 15 minutes – again at fault for incorrectly playing the ball.
Sneyd’s second penalty came on the stroke of half-time although Warrington’s England full-back Ratchford was perhaps a little hard done by being deemed to have ripped possession from Mark Minichiello. It was Watts’s latest wild indiscretion that grabbed the headlines by the end, though.
Hull FC: Shaul; Faraimo, Griffin, Tuimavave, Talanoa; Connor, Sneyd; Taylor, Washbrook, Watts, Manu, Minichiello, Hadley. Substitutes: Bowden, Abdull, Fash, Turgut.
Warrington Wolves: Ratchford; Brown, Atkins, Goodwin, Lineham; Patton, Roberts; Hill, Clark, Cooper, Murdoch-Masila, Hughes, Westwood. Substitutes: Philbin, Akauola, Crosby, T King.
Referee: Ben Thaler (Wakefield)