The pair were members of more than a 100-strong group who were identified as being able to drive yachts along the Spanish coast, despite not having completed, or even taken, the necessary examinations.
Spanish newspaper Las Provincias reports that Valdes failed to explain how his identification was flagged up during the criminal investigation.
Under national laws, all motor boats that exceed four metres in length must have an International Certificate of Competence (ICC) issued, if they are to be driven.
The newspaper also reports that Del Horno, currently a free agent after being released by Levante last season, like Valdes, failed to adequately explain the situation.
Both players have been charged, but not arrested, with the investigation continuing. Valdes is due to feature for the Blaugrana against Real Sociedad on Sunday.