Sanji does not extend his desire to feed hungry people to animals & sea creatures. He appears to offer the sea-cow food, but this is pretty clearly a bluff to lure it into a false sense of security so he can attack it easier. This is a very human-centered way of thinking and I do hope that we see this attitude evolve as the Straw Hats experience character development. While it is very human to differentiate between humans & non-humans, especially when utilizing animals for work, such as when we see them harness the sea monster to tow their ship, it is still an important goal of antifascism revolutionaries to extend the same rights and protections we wish for ourselves to all sentient creatures. This sea-cow likely had wandered off from the grand line looking for an easy meal, and may have attacked the Straw Hats, but it hadn’t shown any violence and deserved the benefit of the doubt. We see this same mentality mirrored by Arlong & the fish people who consider themselves vastly superior to humans and have no qualms about killing them and using them for income. As humans, we have a problem when the fish people subjugate humans, but we don’t have the same misgivings when humans subjugate an animal or other creatures.
The faith that Zoro has in Nami to save him says a lot about both of their character, as well as their already close relationship dynamic. Regardless of how surprised Zoro is to learn of Nami’s background, he is still fully confident that she would not let him die, even to save face in front of these fish people. Furthermore, by testing this in Nami, Zoro is helping Nami reflect on her own choices & attitudes and confront her feelings about the Straw Hats in a more direct manner.
Arlong banning ownership of any weapons and engaging in public executions are both traditional tactics of fascism. He is conquering villages and keeping them under his control to pay financial tributes to his crew so that they can thrive (and pay their own tributes to the Marines) without doing much work of their own. We see them lounging around their estate which is rather luxurious compared to the lifestyles of the villages they dominate. What I find interesting, however, is that there does seem to be some equality amongst the fish people, as we see the crew restraining Arlong and dragging him out of town when the captain begins to attack and destroy the entire settlement. Even though Arlong is letting his temper control him, the crew feels safe & justified to prevent this violent outburst for the benefit of the entire group; if they lose the tribute of Cocoyashi, they will all feel the negative effects of the loss of income, demonstrating that even if these non-humans are violent towards humans, they have respect for each other and their own rules & culture, again mirroring how humans behave amongst themselves contrasting how they treat those they perceive as non-human.
Last note, we witness some anti-tattoo stigma when Usopp is surprised Nojiko is so cool despite her ink. This strikes me as somewhat strange considering the prevalence of tattoos, even on their crewmate Johnny, but isn’t necessarily surprising based on the contemporary attitude of tattoos in the real world, particularly in Eastern culture, in which people assume most people with tattoos are criminals. It also hints at some misogyny in Usopp, as he has never commented on Johnny’s big face tattoo that just says “sea,” which is again common in reality. Some of this can be chalked up to Usopp being inexperienced in worldly matters, having never left his home island before, but the assumptions common in the culture are undeniable, and even in a world full of sailors & pirates, who often have tattoos in the real world, the landlubbers see this as an unfamiliar trait which typically leads to a negative outlook on any behavior.