The 25-year-old centre back, who has 26 caps for his country and made four appearances at the World Cup where Argentina reached the final, signed a four-year deal with the Welsh club after passing his medical on Monday.
Fernandez, part of Napoli's Coppa Italia winning team last season, revealed he sought advice from former Swans striker Michu, who moved to Napoli on loan earlier in the transfer window, and Spanish defender Jordi Amat, who signed for Swansea from Espanynol last year.
"I asked Michu and Jordi Amat about the club. I obviously trained with Michu at Napoli and while I was on loan at Getafe (in 2013) I played with Alvaro Vazquez. He is very good friends with Jordi, so I've spoken to them too," he explained.
"Michu told me it is a family club and the city is very relaxed. He said I was joining a very good club. All I've heard are good things about Swansea."
His move is subject to international clearance but the club say they expect the player to go straight into the squad for Saturday's league fixture at home to newly-promoted Burnley, following their impressive opening day win at Manchester United last weekend.
Fernandez played in both Napoli's Champions League and Europa League campaigns last season but missed both legs when the Italian side knocked his new club out of the Europa League in the round of 32 in the previous campaign.
"I saw a team that holds the ball and plays very good football. It caught my attention," Fernandez said of his new team. "I can identify myself with the Swansea style - I like to play the ball from the back and, of course, I love to defend.
"I was impressed again last weekend with the team's performance and result against Manchester United. It was a very intelligent performance - the team really took its opportunities and it was a fantastic start to the season.
"I just can't wait to start training and playing games."
Fernandez's signing continues a hectic summer for Swansea who have now signed six players, with nine leaving. (Reporting By Sam Holden)