The Moon People engage in trade, and have a fairly amiable relationship with the Storm People (Choumin), since they travel parts of the world that the maritime cultures don't see as much of (such as the vast inland expanse of the continent to the west of where we've been talking about). I'm not sure what category you'd put them in (demihuman, near-human, something else).
The Twilight People (Zath) have a thriving trade relationship with the Storm People. They have things to trade that no one else can make, and guard the knowledge of how to make or fix many of their products (there's a recurring subplot about the effort to keep certain things secret)... they're after exotic raw materials, oddities, and news from afar in exchange. For example, they've figured out a way to make stable rubber, which means they need the raw sap from places where the trees will grow -- but they're also very careful to not let the connection between the sap and the end products get out.
Most of the world's population is human, or from near/variant human groups. There are some others out there, like Fire People in the deep places of the world.
The Grey Ships of the Twilight People are infamous.
No "underwater elves" or such, undersea civilizations seem a bit far fetched outside the more fantastical settings, and this one is a bit more grounded.