Merpeople

Mermen (also referred to as mermaids, though they are both male and female) are underwater people with the lower half of their bodies resembling a fish. They mostly live in the sea in a region called Atlantica. As with fish, mermen must live in the sea or at least in water constantly supplied with oxygen, or else they will die. The Smurfs first encountered a mermaid named Marina in "Handy's Sweetheart" when she was trying to swim upstream to reach the pools of Avalon to find the cure for her ailing father King Aquarius. Though Handy fell in love with Marina, the two of them realized that neither can live long together in each other's world.

Empath: The Luckiest Smurf stories
Although Marina and King Aquarius are the only known characters likely to appear in the EMPATH: The Luckiest Smurf story series, they may not be the only mermen to exist. Marina over the course of her history fell in love with Dylan the Snorkelian, and thus her relationship with Handy was broken off, though they still remained as friends.

Season 10
Oceana, Wave, and Aqua are mermaids that appear in the Season 10 episode "Visiting Atlantica".

Second Smurfette Series
In The Second Smurfette there are many Merpeople who reside in different underwater locations. Marina and Cali are an example of the Merpeople who reside in Atlantica, along with their fathers. Another group of Merpeople are ones who live in Levious, an example are Max and Mere.