The remains of the 28-year-old Italian woman and a 46-year-old Russian man were discovered inside the 12-metre-long (39-foot-long) yacht at the port of Roses on Sunday, a local police spokeswoman said.
"No signs of violence were detected. We are waiting for the results of the autopsy to determine the cause of death," she said.
Local newspaper Diari de Girona, citing anonymous police sources, said the authorities found "huge" amounts of drugs, as well as alcohol and sex toys, inside the yacht which arrived at the port on Saturday.
Roughly a third of the resort town's population of 20,000 are foreigners.
The town is located just across the French border on Spain's Costa Brava, a popular holiday destination for Russians which has also been used as a base by the Russian mafia for money laundering.