Theory: Nami is a mammal

Yes, I have way too much free time for thinking about this. This is just my conclusion. Take it or leave it.

First clue: Nami's body structure. It's so obviously mammalian. The bone structure is all there (for those of you not familiar with this, the bones of mammals are arranged in very similar patterns; the bones themselves just have different shapes from species to species).

Second clue: Nami has breasts. I've heard the "swim bladder" theory, but it wouldn't make much sense for the bulges on her chest to be swim bladders because the swim bladders on actual fish are located close to the spine and inside the body cavity, not on the outside of the ribs as Nami's are.

Now at this point, you're probably still wondering about the scales and tail.

Third clue: Nami's tail. On fish, the tail fans out vertically, but on aquatic mammals such as dolphins and whales, it spreads horizontally. Nami's tail definitely spreads out horizontally. In addition, her tail lacks the "ribbed" structure seen in the tails of bony fish.

So, why does Nami appear to have scales? She's wearing the skin of another animal which has been shaped to fit her. The contrast between her actual skin and her clothing is most prominent around her face and chest. The cut in the chest area of this second skin also provides insight into how a member of Nami's species might slip into it (or struggle, depending on how hard it is for them to stuff their tail in).

Conclusion: Nami is a dolphin-maid.