Although we have previously seen the Marines to be generally corrupt, including the “rat” colonel who both looks like a rat & acts like the archetype often associated with the vermin, we do get an example of their battle prowess here. The single cannon shot flying right over the gates to hit Arlong directly in the face demonstrates supreme skill & ability, even though it is ultimately just a plot device to show how tough & resilient Arlong & the fish people are. Even unprepared, he is able to easily take the blow & stay not only alive but conscious & uninjured. For pretty much anyone else, such a shot would be mortally devastating, but this scene cements Arlong as a stronger opponent than most characters we have seen, some of which may be able to dodge such an attack, but Arlong takes it straight to the face. The fish people’s response of moving the coral reef to create a whirlpool seems like an artistic choice to handwave the physics & leave the audience as confused as the dying Marine, but perhaps I am misunderstanding the scene.
Much like previous villains, such as Gin, Arlong has his own code of honor & ethics that guides his choices, making him a fully-fleshed (pardon the pun) character despite first appearance. As we have discussed before, most people are grey actors without distinctly virtuous or evil lifestyles; like Mary Shelly said, very few individuals choose evil for evil but merely mistake it for the good they see in life. I will not be necessarily surprised if Arlong pulls some devious shenanigans in a future episode but he seems particularly committed to respecting the deal between himself & Nami. This dedication to honoring his word on all matters of money, of course, is not merely altruistic but in fact sets a strong precedent and reputation for dealing with anyone in the future who will believe his negotiations and not try to backstab him before Arlong backstabs them or whathaveyou.
Even though Nami has known the Straw Hats for a short time compared to the years she spent chasing her goal of owning Cocoyasi, she is quickly willing to take some risks to protect her new friends. Every human longs for personal acceptance from their friends & peers, and this crew has repeatedly shown that they genuinely see value in Nami as more than just a navigator so her attachment to the Straw Hats echoes this common desire for kinship and positive social interactions. Even when Johnny is recanting seeing her murder Usopp, Luffy is very quick to deny such things and defend Nami’s honor.
Last note, I know that Sanji’s somewhat creepy “love face” scenes are meant to be slightly comedic & fall within a classic Japanese anime trope, but it also makes sense for his character because he has probably never met a woman before. Working on the Baratie, he would have mostly interacted with sailors & pirates, of which we have seen very few women involved with. Luffy grew up in a port town or something, so he would have seen plenty of women in his life. Besides Kuina, Zoro is very worldly & likely has had romantic adventures in his past. Even Usopp had Kaya to give him some experience talking to a girl. This detail makes Sanji’s cringe behavior towards Nami & her sister contextually understandable.