Of the sentient races that occupy the ocean, the merfolk are the most numerous, or at least visible. Merfolk are as fascinated with humans as humans are with them, most likely due to a physical resemblance combined with the allure of foreign lower body. However, any closer relationship between them is stymied by environmental requirements, though there are many stories of individuals using magical means to overcome that hurdle. Normally a marine race, freshwater variants have been sighted living in especially large lakes.
But the mischievous and playful nature of the merfolk doesn't mean that they're entirely harmless. Humans who abuse their waters or aren't careful with their nets will find their boats more likely to sink, or suffer greater punishment as the merfolk appeal to their own watery gods.
Yay, I got this done just in time for the end of Mermay!
There's a lot of legends about mermaids, but not so many that I've found about mermen. You get plenty of romances between fishy ladies and human men, but not so much the other way around. The British Isles have a whole buncha merfolk stories that mention the male half of the race, but while the females are pretty, the males tend to be less attractive (a design trend that continues to exist to this day). Think of it as the difference between the Zoras and River Zoras in the Zelda games.