This week, a cautionary tale for lusty beachgoers writhing beneath towels: petting and prodding in public, no matter how subtle, could land you on a list of sex offenders.

A couple caught having sex on a Florida beach are facing up to 15 years behind bars on a conviction of lewd and lascivious behavior.

Last July, 40-year-old Jose Caballero and his 20-year-old paramour, Elissa Alvarez, engaged in 25 minutes of sexual acrobatics amidst sunbathers and sandcastle builders on Bradenton Beach, a small town near Tampa Bay.

At some point during their performance, an outraged Bradenton resident whipped out her cellphone to record this hideous public indecency, turning it over to police after the two were arrested.

When the video proved instrumental in court on Tuesday, the anonymous woman recounted the couple’s indelicate display of affection to Fox News, identifying herself only as a grandmother: “The beach was full of families, and it’s like they thought there was nobody there but them.”

Indeed, Caballero and Alvarez had quite an audience.

“I looked at other people on the beach and everyone was watching them but no one was doing anything,” said the camera-wielding granny, who was fielding questions from her grandchildren—curious, confused, and robbed of their innocence. “It was like, shock.”

But shock didn’t preclude voyeurism: adult witnesses stuck around, after all, to observe and assess the couple’s vigorous performance—a nearly half-hour-long display of sexual endurance. (It was later revealed that Caballero was a personal trainer.)

The couple covered up enough that nothing pornographic was caught on camera or reported by witnesses.

When Alvarez finally dismounted Caballero, witnesses say the two briefly cooled off in the ocean before returning to their perch and sacking out for “hours.”

“We really wondered if they were dead,” the grandmother told Fox News. They eventually woke up for a cuddle and then “started into the same thing they were going to do before.”

Another woman who was there with her four-year-old daughter had seen enough. She rang the police, who arrested the couple on the scene and charged them with felony exhibitionism.

It didn’t help that Caballero had recently emerged from prison after serving eight years for another felony charge: cocaine trafficking.

Sex on the beach may seem like a summertime rite of passage. But stringent laws forbidding it have landed many offenders in jail. In May 2013, the Palm Beach Post reported that two people “found naked and having sex on the beach in Manalapan” were promptly arrested.

A year earlier, a couple in Naples who were “intimately involved with one another” on a beach towel were arrested for public indecency, a gateway crime in this instance that led police to charge them for possessing drugs.

Sex in Florida’s sand dunes was apparently so pervasive in 2008 that Palm Beach officials threatened to install cameras on local beaches to discourage public acts of intimacy.

Those tempted to fool around on the beach in glaring daylight should keep an eye out for grannies with smartphones. And if you can’t resist the urge to indulge your exhibitionist proclivities, head for the sea.