This is a list of available professions you might want to take up.
A much more general fighter-type profession, a Weapon Specialist is someone who has to a large or small degree devoted his/her life to the mastery of a single weapon or weapon type, such as Swords, Axes or Whips. This profession applies generally to melee weapons.
The profession of Marksman provides sound experience and insight in calculating range, distance and wind elements and their impact on the path of a projectile. It is a general trait that applies to guns, slingshots, cannons, rifles and the like. People without this profession will have little to no luck in firing cannonballs where they are supposed to go under normal battle conditions.
A Martial Artist is someone who specializes in hand-to-hand combat, completely or very close to unarmed. They usually have good insight in the aspects of weight, balance and movement of the body.
The Assassin profession gives stealth skills, the ability to move silently through areas without being detected. It also gives general poison knowledge, as well as skills with a small weapon such a dagger. An Assassin generally has little actual combat experience, since their victims don't fight back, and as such don't fare that well in a true fight.
The Navigator profession gives knowledge about reading sea charts, currents and weather, reading the stars for a position and a lot of practical ship knowledge, too. Plotting a course along with the related mathematics, all of these are part of the Navigator's job. Navigators will have the skills to navigate the Grand Line. Anyone traveling the Grand Line without a Navigator will be at the mercy of the moderators in their journeys.
The Thief is a job closely related to the assassin. They both provide stealth capabilities, though a Thief gains lock-picking skills instead of poison knowledge, can generally know their way around the ways of barter and trade, and pickpocketing. Thieves don't have it hard to find 'underground info' in most cities, either.
An inventor is a man or woman with knowledge in how various mechanical devices work. They have good insight in how blueprints work, and can upgrade weapons, build explosives and battle engines if they have enough materials.
A Chef, Cook, Bartender or any other distinct related profession gives kitchen skills and knowledge about food, flavoring, ingredients, drink, and nutrients.
A Carpenter is someone who makes a living off building things, from mainly but not limited to, wood. With the times being as they are, most carpenters build ships, but there are also those who instead possess the knowledge of making better buildings and such, as well as smaller items that they craft.
A Smith is a man or woman who makes weapons and tools out of metal. Generally swords, since they fetch the best price, but it is in no way limited to this. Making cannons and parts for inventors is also part of their business, and they also get good knowledge within metallurgy.
A scientist is a different name for those primarily interested in chemistry, the making of explosions and poisons, gas or the like. They get primarily the same basic mathematical skills as the Inventor, and a lot of knowledge about various materials.
A Doctor, on a ship or otherwise, gains large medical knowledge. They also get various other related knowledge in terms of poisons, nutrients, animals, anatomy and certain history. Suturing and cleaning wounds, making bandages out of various materials and preparing drugs, is all part of a Doctor's job. Doctors are the only ones capable of healing serious injuries on the crew.
Of course, if none of these professions are to your liking, you can always create your own custom profession, and the moderators will let you know if it’s suitable. Keep in mind that you can’t just grab any old occupation and expect it to be a profession for the character. There has to be a suitable level of skill required that makes it special. A receptionist is not something that requires some sort of skill that the average person does not have.
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