The 31-year-old man was pulled by fellow bathers from the water off Regueral Beach in Cambrils, Catalonia, and given CPR but could not be revived.
The tragedy happened yesterday afternoon, and the victim has yet to be named.
Local reports suggested the man was swept out to sea after falling from the pedalo and could be seen struggling in the sea.
The local Civil Protection Service said emergency services were called at 5.40pm by a witness on the beach, who said a man was being taken to the sand after he was seen drowning.
It said the man was using a pedalo when he fell into the water and drowned. He was initially given CPR by lifeguards, it added, before paramedics arrived and continued attempts to revive him.
The service confirmed that the green flag was up on the beach at the time of the incident.