I still don't see the appeal. Everyone else was a colorful character of some kind from the moment we spotted them, and many of them further on started to be very unique individuals.
Carrot's got the spirit of a Strawhat, though she isn't unique. Shirahoshi was hella unique (and i was back on team Shirahoshi back in the day), Rebecca too was unique with the "no attacking" angle, though it was quickly apparent how outclassed she was.
What's the draw for Jinbe? His characterization is no different than a number of self-sacrificing leader figures and while his powerset would be unique to the crew, he's one of many Fishmen.
What's the draw for Jinbe? What's the draw for Jinbe?! While such a statement irks me ever so slightly, I understand that your question is genuine and you have yet to see the bluish fat light.
When you say colorful you are probably referring to the eccentric personalities of most of the Straw Hats. Jinbe is certainly tame in that regard but having a 4th Straight Man in a crew of 10 still works just fine especially when you consider his potential reactions to the zany things the other half of the crew does on the regular. And it's not as if he can't join in on the eye popping and face faulting, because he certainly can and did it multiple times like a true Straw Hat. People often want to see the silly side of Jinbe and while his silliness does show in bits and pieces lets at least wait until he is in an arc that doesn't hold such serious weight. It was in his 3rd arc within the series that we finally got to see him lighten up with the face faults and the plan making. You can bet that those are seeds planted for the future.
Now lets talk design...
You bring up both Shirahoshi and Rebecca, 2 characters that shows one of Oda's biggest weakness when it comes to drawing. He makes many of his beautiful girls look just like Nami. From a design perspective regarding the main cast that is just an absolute no go. Each Straw Hat should be easily recognized by silhouette alone and when fully detailed we should be able to pick out details that really speak of the character. Save for Nami(who at the time of entering the story all she really had was darn near being the first girl in the series), each of them has a very unique look to them with physical details that show a ton of thought and consideration. That kind of care did not go into Shirahoshi or Rebecca and well Shirahoshi swimming along side Sunny the rest of her life just wasn't ever going to work.
Now Jinbe on the other hand, Jinbe has the build of a big fat guy! No Straw Hat has that look(or had it for too long anyway) and even when it comes to Fishmen his rounder build is pretty unique. His nose, his hair, his style of dress, all things that stand out among all the other Fishmen and Mermaids. Even without knowing a thing about him, the Sun Pirates brand and scar across his left eye tell us that there is some careful history there with more to be revealed.
Now think about when Jinbe's name first dropped, sometime in like 1999 or 2000 I think. All we knew of Fishmen were that they were super strong racists and Jinbe was in charge of em all! Flash forward a million years and it turns out he is the exact opposite of Arlong and pursues unity between races. Thats pretty unique if you ask me.
As for the appeal, it's really subjective but I love that Jinbe is extremely humble and tries to do everything for every body and more or less fails. When Nami joined for good, I was so happy because I just wanted to see her free and blossom with the crew once her baggage was taken care of. I didn't like Robin at first because she seemed so normal and boring, but once we found out what she was all about I was so happy she was with the Straw Hats because she could basically live free for once. For me much of the same feelings apply to Jinbe. Think about Jinbe and his exploits past or present, for a War Lord he has almost always been depicted as something less than other big shots and almost always failing.
Jinbe is the type that tries to do everything for everyone else at the expense of what he may really want. He puts the weight of the world on his shoulders and seems to always come up short. With the Straw Hats though that would change so much!
Think about it, Jinbe joined the War Lords simply to ease the hostility between races and help the Neptunes. Jinbe can't handle all the kidnappings on Fishman Island, it's White Beard who saves the island. Ace tries to assassinate White Beard and Jinbe isn't able to fully defeat Ace. Jinbe can't keep Arlong in check. Jinbe opposes the war to help White Beard and his actual introduction is of him being imprisoned. Luffy gets a permanent scar because Jinbe can't protect him from Akainu. And while he tried his hardest, both White Beard and Ace ended up dying while he didn't. On top of all that you have Jinbe still conflicted between the ideals of Otohime and Tiger and currently at the complete mercy of an Emperor. Actually I should't say still conflicted because after meeting Luffy he now clearly has a good idea of what he wants to do in life to achieve his dream.
As for fighting styles it's completely unique and visually very interesting! In the water he could be the undisputed king but outside of the water Zoro and Sanji could very well stand above him as they probably have more room for growth. Not to mention the crew would still have some underdog status by not having an ancient weapon at their disposal for the rest of the series.
We as readers do want to see the races of the world at peace with the fish folks actually integrating into human society in full. Not having someone like Jinbe there experiencing those positive changes in the One Piece world as they unfold would be criminal.
A humble, Karate fighting Martin Luther King who sits around trying to name plans and operations is exactly what I want in the crew, don't you?
