The young Magyar's left-foot shot after just 10 minutes proved the difference between two sides eager to secure their place in the second tier for next season.
Buzsaky, on loan to Argyle from European champions Porto, won a free-kick from Brynjar Gunnarsson, took a quick kick short to Tony Capaldi and picked up the blocked shot, rounded a defender, and swept the ball home.
The goal was against the run of play with Watford striker Bruce Dyer having hit the post.
But that was the closest the visitors came despite an impressive performance from right-winger Chris Eagles.
The on-loan Manchester United player tormented Argyle left-back Jason Dodd, himself borrowed by the Pilgrims from Premiership Southampton, but could not fashion an equaliser.
For all Watford's time with the ball it was Argyle who looked more like scoring.
Captain Paul Wotton lobbed goalkeeper Richard Lee but failed to find the target and the same player then hit the side-netting after good work from Buzsaky and substitute Nick Chadwick.
Watford, who played some attractive football, kept going until the end but were unable to give new manager Adrian Boothroyd any cheer as he slumped to a second successive defeat.
Eagles had a free-kick safely held by Argyle goalkeeper Luke McCormick and Wotton headed away another Eagles free-kick from the left.
Another dead-ball from the right, delivered by Ashley Young, also caused some anxieties.
In the end though Buzsaky's early opportunism proved the difference between two sides for whom points are now more important than performances.