Watson, who was the Wembley match-winner against Manchester City in May, put Wigan 1-0 up with a fine first-half finish and three more goals after the break sealed a comprehensive victory for new manager Owen Coyle and his men.
Summer signing Grant Holt netted 10 minutes after the interval to make it 2-0 before Barnsley lost Dale Jennings to a 66th-minute sending off, and Wigan then added further goals in the last 10 minutes through Leon Barnett and Shaun Maloney.
The two other sides relegated from the top flight last season also enjoyed winning starts to life in the Sky Bet Championship, with QPR and Reading both claiming 2-1 home victories.
QPR, considered favourites for promotion, fell behind against Sheffield Wednesday when Atdhe Nuhiu netted in the 19th minute but Harry Redknapp's men bounced back to claim the spoils with two goals in three minutes just before half-time through Nedum Onuoha and Andrew Johnson
It was a memorable day for promoted Yeovil who, playing at this level for their first time in their history, claimed a 1-0 win at Millwall thanks to Edward Upson's late strike.
Bournemouth, who came up from League One alongside Yeovil, were also celebrating a win as a brace from Lewis Grabban earned them a 2-1 home triumph over Charlton.
Charlton had equalised through Yann Kermorgant's stunning scissor-kick but their hopes of getting anything out of the game suffered a blow when Bradley Pritchard was dismissed.
League One champions Doncaster could not complete the set for the division newcomers as they lost 3-1 at home to Blackpool.
Doncaster had looked to have salvaged a point when skipper Rob Jones (60) levelled up Steven Davies' 17th-minute opener for Blackpool, but two goals in the last two minutes from Gary MacKenzie and Tom Ince earned the visitors the spoils.
Last season's losing Championship play-off finalists Watford began the new campaign with a narrow 1-0 victory, Troy Deeney's 11-minute strike proving the difference against Birmingham, while a 52nd-minute goal from Henri Lansbury earned Nottingham Forest a 1-0 home win over Huddersfield.
New signing Luke Murphy made an immediate impact for Leeds as he netted a last-gasp winner to earn his side a 2-1 comeback victory over Brighton. The Seagulls went ahead early on through Jose Ulloa but Ross McCormack drew Leeds level before 23-year-old Murphy netted a 94th-minute clincher.
Leicester also came from behind to beat Middlesbrough 2-1 in an an entertaining game at the Riverside. Lukas Jutkiewicz put Boro ahead in the first half but Leicester struck back with two goals in seven minutes midway through the second half from Danny Drinkwater and Jamie Vardy
Meanwhile, Burnley and Bolton shared a 1-1 draw in the day's early kick-off at Turf Moor, Darren Pratley's sweet first-half strike earning the visitors a point after Danny Ings' deflected effort off David Wheater had put the Clarets ahead.
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