Travel writing
Melville was best known as a travel writer—it's what he was most popular for. People weren't all that into the weird allegory about the giant white whale, but they liked it when Melville did the National Geographic thing and showed them cool new places they'd never been. "The "Encantadas" is, then, Melville doing the sort of genre writing that made Melville into Melville.
It also shows, though, the extent to which Melville was maybe bored with travel-writing Melville. Rather than a cheery travel writing story about a lovely place you might want to visit, "The Encantadas" is all about the Galapagos as awful nightmare spot; a place to have the worst honeymoon ever, (and of course, Hunilla does have the worst honeymoon ever, just about.)