It had looked as if the Reds would take all three points after twice coming from behind, new signing Mohamed Salah eventually making it 3-2 in the 58th minute.
However, the visitors' set-piece issues arose once again, Britos bundling the ball in from a late corner to level.
Hornets off to a flyer
With the joint-fastest opening Premier League goal having been scored the evening prior, the chances of another early goal looked unlikely. Watford though, thought differently.
It was from the resulting corner though, that the Hornets took the lead. With Dejan Lovren and then Roberto Firmino failing to track the big man Okaka, he easily breezed towards the near post to power a header past Mignolet.
A goal each in frantic five minutes
Liverpool would grow frustrated chasing an equaliser, but they did finally get one just short of the half-hour mark.
Sadio Mane had looked quiet but burst into life as he instigated a cute one-two with Can, driving into the box and bending a delightful finish past Heurelho Gomes.
Unfortunately for the visitors, they were only level for a matter of minutes, some comical defending allowing Watford to go back ahead.
All coming from a throw-in down the right, Jordan Henderson's failure to track Tom Cleverley allowed the new signing to cross after the ball broke, with Abdoulaye Doucoure taking advantage to slam the ball home after Trent Alexander-Arnold's clearance rebounded off teammate Matip.
Chances did come before the break for the away side to level up, new signing Mohamed Salah blazing over after being put through at a tight angle, whilst a Mane header fell just wide on the stroke of half-time, the winger unable to pick up his second from Firmino's in-swinging corner.
Salah went closer than his previous efforts just after the restart, bending one narrowly wide with his favoured left-foot after being played it by Georginio Wijnaldum.
Salah and Firmino fire Reds ahead
That miss mattered little a few minutes later though, as Salah played a pivotal role in the Reds' quick comeback.
Starting with the equaliser, Salah using his pace to get free in the box brought Gomes rushing out, the Egyptian nudging the ball round the 'keeper before being brought down. With James Milner on the bench, Firmino stepped up to send Gomes the wrong way.
Liverpool weren't done there though, going ahead just minutes later as Salah got one of his own, set up by Firmino. Lovren's ball down the left caught Watford cold, Firmino skipping away from the defence to lift the ball over Gomes but a little wide of the goal, Salah on hand to get there first and tap in.
Hosts seek to mount late charge
The Reds nearly had three goals in 10 minutes as a Joel Matip header came down off the under-side of the bar, bouncing out after Alexander-Arnold's inviting corner.
In amongst the defensive collapse that he was overseeing, Marco Silva took the opportunity to introduce record signing Andre Gray, in place of Okaka.
That change didn't immediately halt Liverpool's pursuit of more goals, Lovren taking his turn to head at goal from an Alexander-Arnold corner, this one well saved by Gomes.
Watford went well in search of an equaliser but were still being heavily threatened by Salah on the break, Gomes impressively on his toes to cut out a promising through ball from Can towards the new signing.
The hosts finally got their chance in five minutes of stoppage time as substitute Joe Gomez gave away a free-kick on the touchline. Failing to clear fully, Watford kept it alive with Miguel Britos forcing Mignolet into a smart save for a corner.
That corner wasn't dealt with, Britos capitalising after a poor header from Wijnaldum, the ball nodded across goal by Richarlison and bundled in by the defender, amid accusations of offside.
Those shouts weren't Watford's concerns though, with the dramatic final goal ensuring they took a point from Liverpool.