This is because there are several social classes, and many situational uses I in Japanese.
When you study Japanese, you may find the word 私（わたし) "I" in English as the most common translation of I, but we use more than 私（わたし) .
Japanese pronoun is unique because you can tell a persons personality by the pronoun that they choose to use.
Using characters from One Piece let's figure out who uses which pronoun. People may use many of these themselves, you too can choose your pronoun of preference depending on your personality.
*Japanese 'R' sounds more like 'L', but the Japanese government uses R for らりるれろsounds when they show Japanese words in foreign languages, so I too use R.
Who uses 私(わたし, watashi) or (わたくし,watakushi)?
Nami, Robin, Brook (he uses わたくしsometimes)
私(watashi) is used for usual women, girls, and men for business situation, and men who are or appear to be royalty. I use 私 in business, shool, and family situation. My dad uses 私 in business situation, but not in family.
Who uses 俺（おれ, ore）?
Luffy, Zoro, Sanji, Usopp, Chopper, Franky, Shanks...a lot!
Ore is used by many guys who look/try to be/want to be strong, bad, fashionable and cool.
This is used only among friends, family, and playful group.
A lot of Japanese men use it except in the business scene.
Who uses 僕(ぼく, boku)?
Cobby, many kid characters.
Boku is often recommended to use for kids by their parents specially in school.
Some Japanese men still use, but sounds more weak, gentle, and serious like Cobby.
When elementary school kids write an essay, they use boku mostly as their pronoun.
Who uses わし(Washi)?
Garp (Luffy's grandpa), and many old characters.
I've heard people who use washi as their pronoun only few times in my life (in real life), but a lot of anime characters specially old people use washi. According to Wikipedia, washi is still used by old people in Hiroshima. Mayby it is part of the Hiroshima dialect.
*The following, I have never heard in real life.
Who uses わらわ(Warawa)?
Sounds very old style. It was used 300-500 years ago in real life, but not any more.
Hancock is living in an isolated island, so the author may want it to appear that the island not changed for a long time, or isolated from outside influences.
Weird pronoun. It makes him sound like a more strange character.
Same concept as Achishi...
By the way, Son-Goku from Dragon Ball uses 'Ora', because of this Ora has become a popular pronoun in Japan among boys. This made some parents worried because Ora is used only in anime, and it sounds funny, naughty, and tomboyish. Growing up my brother used it for a year, before my mum put a stop to that. Hahaha...
Have you decided which pronoun you will use?