Jinbe is part of a race of fish people who has been repressed and enslaved by humans for hundreds of years. Like many other fishmen Jinbe went out to the sea to become a pirate to enslave humans and dominate the seas because in some weird twist they consider themselves superior to humans. And on average probably also are physically superior, can breath under water and are of course extremly fast.
Jinbe is probably the most powerful fishman alive. He realized the wrongs of his way and eventually joined the world government and was pardoned. This and a couple of other political deals finally lead to an abolishment of fishman repression. However, further disobiedience over minor infractions of slavery led to a fallout with the government and he was finally imprisoned until Luffy coincidentially destroyed the prison he was in and set a couple of the most dangerous criminals in the world free.
By the time he was still a pirate he had a formidable 400.000.000 berry bounty and his application to the Strawhats is still pending. He'll do fine as a reserve player.

https://www.youtube.com/watch?v=1MifVy8UEj0