Bystanders can reportedly be heard shouting at the man, with one yelling: "Go on, go back home".
Another onlooker apparently shouted: "He is stupid. He wants to die".
A taxi driver told local media: "A young man threw himself into the water and two or three life jackets were thrown from the boat but he refused the help."
A fellow cabbie added: "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 said: "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."
More than 181,000 migrants arrived in Italy by boat in 2016, most of them from sub-Saharan Africa, an increase of almost 18 per cent compared with 2015.
Authorities are now investigating the incident which police say happened on Sunday.
Italian media said Sabally had arrived by boat two years ago and had residency papers for Italy.
- with Daily Mail
