Romelu Lukaku scored twice for Everton, bookending the scoring, but Okaka's double Walter Mazzarri's men victory.
Italian striker Stefano Okaka's first two Watford goals inspired a 3-2 comeback win over Everton on Saturday, leaving Ronald Koeman's side with one win in 10 Premier League games.
Romelu Lukaku scored twice for Everton, bookending the scoring, but Okaka's double -- the first a brilliant flick -- and a Sebastian Prodl header earned Walter Mazzarri's men victory.
It was Watford's first win over Everton since a 2-1 success in March 1987 and took them above the Merseyside club in the table, putting them level on points with Manchester United.
Everton have gone five games without a win and now face a daunting pair of home games against Arsenal and derby rivals Liverpool before a trip to champions Leicester City.
Everton received a blow this week with the news close-season signing Yannick Bolasie is likely to miss the rest of the campaign due to a serious knee injury.
Gerard Deulofeu took over from the former Crystal Palace winger at Vicarage Road and drew an unorthodox, volleyball-style save from Watford goalkeeper Heurelho Gomes with a teasing early cross.
The hosts seemed to be building up a head of steam, with Ramiro Funes Mori obliged to slide in and prevent Okaka reaching Etienne Capoue's low cross, when Everton took a 17th-minute lead.
Gareth Barry did not even need to look up to know where Lukaku would be running, hoisting a first-time pass over the Watford defence for the Belgium striker to finish past Gomes.
Okaka's first attempt at an equaliser lacked the pace or placement to trouble Everton goalkeeper Maarten Stekelenburg, but his second effort was a gem.
Nordin Amrabat curled a cross into the box from the right and Okaka -- signed from Anderlecht in August -- stole in front of his marker before pirouetting to flick a graceful back-heel volley home from six yards.
Watford might have gone ahead before half-time, but Troy Deeney miscued after ghosting in to meet Adlene Guedioura's deep cross.
The second goal would not elude them for long, however, as Jose Holebas bent a free-kick into the box from the left in the 59th minute and Prodl charged in to plant a header past Stekelenburg.
Within five minutes it was 3-1. Stekelenburg produced a finger-tip save to keep out Miguel Britos's header, but from the resulting corner Holebas picked out Okaka to glance in his second goal.
Koeman sent on Ross Barkley, Enner Valencia and Aaron Lennon and the latter's left-wing cross allowed Lukaku to reduce the arrears with a close-range header four minutes from time.
But Watford held on, a stoppage-time Barkley free-kick the only meaningful effort Gomes had to save.