Police used palm trees to mark off the boundaries of a 100-meter (100-yard) expanse for the American delegation. On either side, onlookers gawked.
As the first lady rested inside a canvas hut by the shore, her 9-year-old daughter splashed around in the sea and a security guard swam with her.
The Obamas arrived Wednesday for a five-day stay in southern Spain that is being described as private, although Mrs. Obama is to meet with King Juan Carlos and Queen Sofia Sunday at their summer residence on the island of Mallorca.
On Thursday they toured Granada, taking in the Alhambra palace - the exquisite reddish citadel that was the seat of Moorish rule in Spain - and the city's cathedral, and also watched a flamenco show in a hillside cave turned into a tavern.
At that performance, the first lady clapped along with the booming chords of the guitarists and rapid-fire, clicking heels of the dancers.
"She did it well. This woman has rhythm," said Juan Andres Maya, leader of the flamenco troupe.
Some Spanish media outlets have covered the visit in extreme detail, reporting on the first lady's every move - from where she window-shops and what she buys to what flavors of ice cream she and her daughter had before going into the cathedral in Granada (for the record, chocolate for Mom, melon and raspberry for her daughter.)
President Obama and the couple's elder daughter Malia stayed in the United States.
The newspaper El Mundo ran a lengthy story Friday on the Obamas' dinner their first night in Spain. Sasha ate pasta, and her mother and the rest of the U.S. delegation had tapas that included sea bass tartare, strawberry gazpacho and sardines, followed by a main course of lobster with seaweed risotto.
The meal cost about euro50 ($66) a head, the paper said.
The Obamas have also been showered with gifts, including a typical Spanish fan, a traditional veil known as a mantilla, books on the cities they have visited, and a copious assortment of local food including cheese, olives, wine and even two legs of salt-cured Spanish ham. It is questionable whether the ham will clear U.S. customs because the producers lack permission to export to the United States, media reports said.
While many Spanish media picked up a column from the New York Daily News that complained that Mrs. Obama is taking an expensive vacation while many other Americans struggle with hard times, the coverage here has been gentle.
There has been at least one case of grumbling, though it targeted the Spanish gift-givers rather than the American recipient.
Salvador Pendon, a local Socialist official, said that since Mrs. Obama is here on a private rather than official visit, it is not right for Spain to deluge her with so many goodies.
"It is too much stuff. And all these gifts mean just one thing: the givers are trying to draw attention to themselves," Pendon said.