1. Jinbe doesn't need to have a job to join, Usopp, Zoro and Robin didn't. And Chopper was invited before Luffy knew he was a doctor.
2.If he absolutely MUST have a job, why is everyone so hell bent on helmsman (ship steering) for no apparent reason, when his much more obvious job would be lifeguard? The guy who dives off the ship to to save the guys that can't swim?
Failing that there's still Striker and Boatswain.
Often overlooked, the Striker was a native of the West Indies , typically from Darien or the Mosquito Coast . They were expert hunters who trapped sea turtles and manatees; fished for sharks and other large fish; and also hunted wild game when the crew came ashore. Their knowledge of local plants aided in collecting edible fruits and vegetables as well as medicinal plants and herbs. Their expert ability at hunting and fishing earned them a spot among the crew, Their hatred of Spain assured their loyalty and ferocity in battle. They were not kept aboard for their seamanship, their job was to catch fish and kill Spaniards.
Boatswain / Bos'n / Bosun
This position may be compared to the modern chief petty officer. The Boatswain supervised the maintenance of the vessel and its supply stores. He was responsible for inspecting the ship and it's sails and rigging each morning, and reporting their state to the captain. The Boatswain was also in charge of all deck activities, including weighing and dropping anchor, and the handling of the sails. A ship of any size would require the boatswain to oversee several junior officers who would share his responsibility for the crew's morale and work efficiency as well as the maintenance and repair of the hull, rigging, lines, cables, sails, and anchors.
Nami, Sanji and Franky perform those roles in some capacity, but… catching fish, supervising the younger unruly crew, and LIFEGUARD are all much more in keeping with his actual role than... steering the ship.
@jalle153:
I believe that neither Jimbei or Vivi can join. The reason is that both of them have an important position in their respective countries.
Vivi is a princess that has to rule her country.
Jinbe is an outlaw highly wanted pirate that can't step foot into his that has no actual remaining responsibilities other than tying up the loose ends of an old deal with Big Mom.
Somewhat different.