Currently, I reading Death's Master (part of the aforementioned Tales from the Flat Earth). The hero Zhirem is currently captured in an undersea kingdom. That reminded me of my take on mermaids.
Mermaids are of course no gentle creatures. They are murderous monsters bend on drowning and devouring sailors and other unfortunate men. So lets build a mermaid. Again, I am going for the lower end of the hit dice spectrum. Merman are usually given 1 HD. But I find that a bit underwhelming. So lets settle for 3. Enough to be a more serious threat. Armor class is probably going to be 12. They are unarmored after all. Let's add one more point for higher dexterity. They will be very fast in water, but not so much on dry land... Morale will be above a snivelling creature (6) but below a courageous monster (9). So I am settling for an eight. Damage will be by sharp teeth or claws. Somewhere around d6. Of course they are able to breath water and air. Also, they can cast charm person at will (instead of an attack). But the spell will only work on males.
All in all, nothing special here. They want to drown and eat you, so better forget any romantic notions about live under the sea.
Mermaid: HD 3, AC 13, Speed: double that of a walking man in water, practically nil on land, Morale 8, Damage: d6, Able to breath in water and air, charm person at will, AL: Chaotic.