It is known that there are "human" figures who live below the waves. Whether these are nereids and tritons, offspring of nereids and tritons, crossbreeds twixt these spirits and humans, or a race in their own right, is discussed vigorously by sailors and by sages (most of which would never deign to set foot on a ship).
People living inland lack any direct knowledge of the merfolk, but they nonetheless figure in tales and art everywhere.
The seaborn are held to have the upper bodies of men and the lower bodies of fish (unlike the orthodox depiction of nymphs but like tritons). Often their lower bodies and hair is portrayed as blue or green. There is a disagreement whether merfolk have one or two tails.
Mermaids (who receive most of the attention) are portrayed as savage and wanton creatures, with an enormous appetite for men. However, this appetite is said to extend beyond tumbling them, so that the mermaid will attempt to drown her paramour after copulating, and bring him to the depth for consumption. "Mermaid" is a widely euphemism for harlot, and given their vain and wanton ways, art depicts mermaids with naked breasts and pudena, often with a comb and mirror.
First-hand knowledge of merfolk