"It was terrifying," she told The Sun.
She added: "There was another lady swimming on her own the other side and she was totally oblivious to what was going on.
"Its tail was flapping around and it looked distressed, which wasn't nice to see."
It swam off moments later as hundreds of beachgoers looked on in shock.
Nobody was hurt when the 8ft shark - whose species has not been identified - swam among the tourists.
Many species of shark can be found off Spain and in the Mediterranean Sea, including blue sharks, thresher sharks and even great white sharks.
But they are very rarely spotted and interactions between swimmers and the large fish are rarer still.
On Wednesday, however, Spanish media reported that a boy was savaged by a shark off the country's island of Gran Canaria in the Atlantic.