At least three life rings were thrown into the water near the man, who was named as Pateh Sabally, a 22-year-old Gambian, but he did not appear to reach for them, raising speculation he wanted to commit suicide. No-one jumped in to help him.
Police said the incident happened on Sunday.
A video recording shown on various news websites appeared to pick up some bystanders yelling at the flailing man. "He is stupid. He wants to die," says one person.
A taxi driver told local media:
'This is despair. I do not know the story of this guy. They have said he is African. Who knows what he's been through, how many seas and deserts he has crossed? And he came to die in the most beautiful city in the world.'Witness Abdus Sattar from Bangladesh confirmed that the drowning had seemed to be a suicide.
'He threw himself from the pier. At first I thought he slipped but when I saw they threw lifebuoys and saw that he refused, I realised that he had decided to call it quits.'Italian media said Sabally had residency papers for Italy.