A delightful fuck-ton of mermaid anatomy references.
A bit of a talk-through; Consider how natural swimming is to mermaids/men. When swimming, they often won’t be utilizing their hands and arms to help them move forward, ‘cause they’re adept to using their tails and want to keep their arms free for multitasking. Of course there are situations where it’s required, like when fighting currents and twisting around. Because they’re swimming on a horizontal, their heads will be turned upward, just like if we humans were standing vertically looking up to the sky. Mer-peoples’ chests are often puffed outward, because the arms are typically pulled back behind them (just like when humans are told to “sit up straight and show good posture” by rolling the shoulders back). Do keep in mind that the GIFs above don’t show full tail movement, as human legs don’t bend outward; mer-tails will be like the side-to-side fish swimming motion, but flipped to the side (if that made any goddamn sense). Don’t forget they’ll be breathing underwater (and maybe on land, depending if you want to grant them that ability)! Are you giving ’em gills, or are the human lungs somehow acclimatized to breathing water? Think about whether you’re designing a mermaid for practicality or aesthetics; if it’s gonna be practical, their hair will more than likely be cut short or tied up, ‘cause, though swimming with long hair isn’t too difficult, it tangles wicked easily and can impede sight (in the ocean, a mere second can cost you). Of course we all like the long flowing hair (I do too), but that’s just something to think about. (Be aware that good haircuts on land might not be good underwater, and vice versa. Consider how the water will affect the hair movement.) And designing the fins, too. Think about whether you’re splicing them with another sea creature or if they’re a new design entirely. Think about how you want them to swim and how the fins will help that work. Also, depending on how “cultured” you want ‘em to be, they probably won’t be wearing clothing, because no other creature under the sea does. But, then again, they’ve been in contact with humans (otherwise there would be no myths and rumours), so maybe they took after us. Who knows. Just food for thought. (Merpeople can be creepy as fuck, too; they don’t have to be these gentle maidens.) And of course, there’s the reproductive/excretive system. If we assume mermaids take after whales, the genitalia will be just about ”mid-shin” level in front. Some people put it at the end of the tail (between the two fins), and some make the fish half disappear slightly at the waist to expose “human” genitalia. You can get creative with it, since they’re mythical creatures. Maybe their belly-buttons are also anuses. Who really knows. “What about merman genitalia?” Look up “male whale genitalia” on Google, or “male fish genitalia.” That might help.
[From various sources]