Nicolas Faucon said he was underwater untangling the anchor chains of boats on Saturday morning when he spotted the creature.
"A shark passed by just 50 centimetres from me. I'm 100 percent sure," he told the newspaper. "An animal like that is easy to recognize. It was more than two metres long but wasn't aggressive. It just looked at me and changed direction."
Shocked by the encounter, he quickly resurfaced. His wife happened to be waiting for him and told the newspaper he was visibly shocked. "He was panicked and immediately told me what had happened," she said.
Faucon alerted the authorities who carried out a search, but the shark had apparently disappeared.
Reassured, he went back down into the water, only to meet the shark again an hour later. "This time, I got the feeling the shark was scared when it saw me. It left quickly."
Faucon saw the shark for a third time on Sunday morning, around the same time. He said he now feels nervous about going back into the water. "Three times in a row. That's not trivial. But I have to do my job."
August is height of the French holiday season with thousands of tourists from around the world hitting beaches along the French riviera.
Shark alerts have been raised in the waters off the riviera before, although they have often been false alarms. Beaches were closed last August after a suspected shark sighting, although there has only been one fatal shark attack in French coastal waters since the 1840s.