I guess the character I have been calling “Octojohn” is actually named “Hatchan” so that is a little funny.
The fish people’s sense of superiority is pretty clearly just an analogue for racism. There have been a number of allusions to this point in previous episodes but it is exceptionally clear here. Beyond Arlong’s crew treating human settlements as cattle to bring them unearned income, or even comments about how pathetic it is that humans cannot breathe underwater, we see Hatchan repeatedly state things about how weak human-made swords are and that he is a better swordsman than any human because he has 6 arms, even though Zoro is able to defeat him while gravely injured & fighting a fever. This mimics real world racism that will often focus on real, but arbitrary, genetic differences between people, such as hair or skin color, different facial features, or other minor traits. There are also mildly consequential things that bring about racism, such as lactose intolerance, sunburn resistance, or even cultural elements, but none of these deserve hatred or disdain, including if someone were to have some extra arms (or less arms).
Zoro is so skilled in combat in no small part due to his mental fortitude and sheer determination. He has made promises & set goals, so his memories of these events push him to persevere because there are people other than himself depending on him to secure victory. This confidence is an advantage but also a heavy weight that he bears in all situations he crosses. Zoro is motivated not only to succeed & win but to truly be better than anyone else without making excuses.
Last note, I really appreciated how Nojiko & Genzo were able to communicate so clearly & efficiently while underwater. Their use of facial expressions & simple gestures to convey complex meaning demonstrates their close relationship & long past living together in Cocoyashi village. They have a shared culture and similar lived experiences, giving them access to similar thoughts & ideas that formed their personal worldviews. Even if they are not best friends or anything like that, they can still get their points across with minimal communication options available.