I'd like to discuss the innate potential of Fishmen. As a race, the Fishmen are said the be on average 10 times stronger than Humans. Now of course we know this is just the basal strength. We've seen that among humans in the OP world, the variation in strength is so drastic it's almost inconceivable.
So far though, I think we can safely say that the strongest Fishman we've encountered is Jinbe. Yet, while Jinbe is certainly a top tier fighter, he's definitely not at a level where he can combat someone like Sakazuki. Also, Jinbe has been shown time and again to be pretty unique. No other fishman alive even comes close to his abilities. Hody's gang was said to be a joke compared to Jinbe in terms of their mastery over Fishmen karate, and just raw power alone. Even Arlong, who was Jinbe's peer is way beneath him. In fact, the only other fishemen we've seen that could arguably be compared to Jinbe is obviously his captain Fisher Tiger, who is unfortunately dead. (I actually have a side question about this. Can we say for sure that Fisher Tiger is stronger than Jinbe? I've always had the feeling that Jinbe has always been the strongest. He simply respects Fisher Tiger as a hero and good leader and willingly follows him.)
So here's my question. Why aren't there more strong Fishmen? If the fishmen's basal potential is already 10 times higher than humans, logically shouldn't it be easier for Fishmen to get that much stronger? Why isn't a top tier fishmen 10 times stronger than a top tier human then?
Here are some possibilities:
1. Population. The human population is just waaaaaaay bigger than Fishmen. Just from population alone, the odds of having strong humans ended up as such, with them far outnumbering strong fishmen like Jinbe.
2. The 10 times stronger isn't actually true. It's a lie the fishmen came up with to make themselves feel better
3. The 10 times stronger refers to the most basic of humans vs fishmen, i.e. the average joes, and doesn't actually scale upwards. In fact, while normal fishmen are stronger than normal humans, the 'maximum potential' of fishmen is actually lower than humans, where fishmen of Jinbe's calibre is probably the highest they can achieve.
4. The 10 times stronger refers to average joes, and it DOES scale up to a certain extent. BUT, the super strong humans we know of are not part of this range of 'normal humans'. Meaning these really strong people are almost like a subspecies of humans, or rather special exceptional humans who just have an overall much higher potential than regular humans. In other words, this is similar to how a portion of our popuation are just born to be geniuses that excels at something beyond regular humans. Just that in the One Piece world, this 'exceptionality' often translates to fighting ability. In the case of Fishmen, such exceptional individuals do occur from time to time, and Jinbe is an example. Simply by virtue of population differences, the odds of having 'exceptional' humans ended up being that much higher than 'exceptional' fishmen (in this case 1?).
5. The mindset that they are born 10 times stronger than humans worked against them. As a result, fishmen generally do not train and work as hard as humans do in improving their abilities. Jinbe is just the product of exceptional hardwork. Considering how Jinbe is said to have mastered the fishmen karate to the highest level, it suggests that other fishmen could have mastered the fishmen karate better, but just didn't.
6. 10x stronger refers purely to physical strength. Many top tier fighters are strong for other reasons apart from physical strength. Having said that though, we've seen some humans show ridiculous physical strength, moreso than any fishmen. In fact, these really strong fighters tend to have monstrous strength as well (like Luffy) that go way beyond what their body size would suggest. Physical strength contributes a lot to fighting ability, and the top tier fighters tend to be physically strong as well, and are seldom fully dependent on their abilities, even if they are ability users.