Santon returned to first club Inter Milan on a permanent basis last summer, following an initial loan spell.
However, the 25-year-old has made just 10 Serie A appearances this season and was linked with January moves to a number of English clubs.
Newcastle were tipped to re-sign their former player on loan, however the Chronicle reported that Santon was only interested in a permanent deal.
Watford, who look certain to remain in the Premier League, do not yet know whether Chelsea left-back Nathan Ake will return to Vicarage Road following his loan spell and Santon could be a good option to replace the Dutchman next term.
The Italian can play on either side of the back four and at centre-back, if required.
Right-back Allan Nyom has been a regular for Flores' side this season, though left-back Jose Holebas has found first-team opportunities harder to come by.