During a week when the midfielder's future at Goodison Park came under intense scrutiny, he proved the difference as his side overturned a run of two straight defeats.
Ronald Koeman's team were the first to venture forward, and they almost took the lead in the sixth minute when Barkley tested Heurelho Gomes in the Watford goal with a low strike towards the bottom corner.
Gomes thwarted Everton again in the 13th minute as Leighton Baines's corner was taken on the volley by Phil Jagielka, the Brazilian producing a vital block at his near post to keep the scores level.
Youngster Tom Davies was next to test Gomes, covering plenty of ground during a mazy run before pulling the trigger, unleashing a piledriver that the Hornets goalkeeper could only parry at the first time of asking.
Everton netted their decisive goal in the 56th minute when Barkley latched on to Jagielka's well-timed pass, outmuscled Sebastian Prodl and drove the ball beyond Gomes from 25 yards to make it 1-0.
Watford's best chance to equalise came in the closing stages when Okaka gained possession inside the box and pulled the trigger at close range, forcing a reaction save from Robles.
The result was meaningless in terms of Everton's final league placing given that they could not move higher or lower than seventh before kickoff, while Watford need one point from their remaining two games to mathematically cement their place in the Premier League for another season.