The couple were swimming with a red float, appearing fine one minute and under water the next, witnesses told CNN affiliate WSVN.
Beachgoers along with Ocean Rescue guards rushed to pull them from the water, according to WSVN.
"They were in cardiac arrest when fire rescue arrived," said Capt. Adonis Garcia, spokesman for Miami Beach Fire Rescue. "We worked them all the way to the hospital at Mount Sinai when they were pronounced dead."