New manager Quique Sanchez Flores will kick off a first campaign in English football away at Everton, having been appointed following the Hertfordshire club's promotion.
Watford finished rock bottom when they were last in the top flight in 2007, but have already made moves to strengthen the squad.
Austrian defender Sebastian Prodl and Lithuanian goalkeeper Giedrius Arlauskis have been signed, while a move for £6million-rated Blackburn striker Rudy Gestede has also been touted along with a deal for Genoa's Argentinian winger Diego Perotti
Barnes spearheaded the Hornets' attack when they finished runners-up in the old First Division under Graham Taylor and he also reached the 1984 FA Cup final.
However, the 51-year-old former England international believes Watford need to have plenty of strength in depth if they are to last the distance among the elite of English football.
"They need to strengthen in all areas
When teams come up from the Championship, the gulf is huge," Barnes said in the London Evening Standard.
"They can't think they have a good enough defence
They need to be realistic
There's no point spending millions and millions because the best players won't go to Norwich or Watford.
"They need to get better quality all-round players, but they shouldn't spend money for the sake of it.
"They need to get solid, hard-working players and solid defenders with someone who can score goals like Burnley did with Danny Ings."
Sanchez Flores cannot wait to get started, but Barnes believes the Spaniard, who won the Europa League with Atletico Madrid, will face a steep learning curve.
"It is going to be challenging managing anyone in the Premier League," said Barnes, who had spells coaching at Celtic and Tranmere, as well as with Jamaica's national team.
"He was obviously at a very, very good club challenging for the Europa League (in 2010) although not in the league all the time
Keeping Watford in the Premier League is a completely different scenario.
"He is new to it and he won't know much about the players, so he has to be given time.
"I think he's a good manager and if he knows the players he wants and can get them, he has a chance.
"Fighting off relegation is something he will be doing now."
Sanchez Flores, appointed after Watford decided not to renew the contract of Slavisa Jokanovic, is determined to get things right at Vicarage Road.
"Seeing the Premier League fixtures and knowing I will be part of this with Watford makes me want the start of the season to be here tomorrow," he said on the club's official website.
"I have a chance to play a big part in this with the Watford players and the fans
This is the most important moment for me now, to prepare and be ready for an amazing challenge in my career."
Striker Troy Deeney, who scored 21 goals as Watford finished second to Bournemouth, has emerged as a target for Newcastle, whose new boss Steve McClaren tried to sign him at Derby last season.
Source : PA
Source: PA