The ferry supposedly targeted carries up to 2,400 passengers and sails twice a week, EFE news agency reported.
ETA said last month it was calling off the year-old cease-fire with the government because of failed peace talks.
The armed group said in a statement it would resume its campaign "on all fronts," El Pais reported online, citing Basque newspapers.
Spanish Prime Minister Jose Luis Rodriguez Zapatero conceded at the beginning of 2007 the peace process with ETA was not proceeding as he had hoped.
ETA militants have been fighting for three decades for a Basque homeland along the French-Spanish border. Their attacks have claimed some 850 lives.