Apart from two consolation goals after the Championship side took their foot off the pedal late in the game, the Saints who were never really in the hunt.
Giantkilling seemed to be on the cards from the outset as Bruce Dyer twice went close before finally finding the net after 39 minutes.
Dyer headed firmly into the net from Neal Ardley's cross, but the visitors' marking was atrocious.
Six minutes into the second half following another corner by Ardley, James Chambers was there at the far post to plant the ball home in a crowded goalmouth.
Watford continued to press and in a sense let Southampton off lightly by only scoring five.
Dyer showed tenacity on the left working his way through two clumsy challenges before pulling the ball back for Chambers to ram in the third from 12 yards out.
Southampton were reeling four minutes later when Heidar Helguson scored the goal of the game. Coming in from the left, he met a chipped ball from Gavin Mahon to send the volley beyond keeper Alan Blayney into the far corner of the net.
With a 4-0 lead, Watford began to lose concentration and Saints substitute Dexter Blackstock, demonstrating more tenacity than most of the starting line-up, went through to meet a cross from Leandre Griffit and steer it home from close range.
Almost immediately Watford added a fifth. A weakheader in defence set up substitute Hameur Bouazza who rifled in a left-foot shot from ten yards.
Two minutes from time the visiting fans were given a crumb of consolation when Danny Higginbotham crossed for Brett Ormerod, the most determined Southampton player on view, to head in.