The 1980s cartoon series The Smurfs has one episode where the Smurfs meet a mermaid.
In "Handy's Sweetheart", Handy Smurf is using his diver's suit in the river when he sees a mermaid in trouble. Handy brings the mermaid to the surface, setting her up in a portable tub of his own invention that helps her to breathe above water. The mermaid, Marina, traveled far from home to obtain a special lilyroot that her father needs because he is ill. The Smurfs come to the rescue, vowing to help fetch the lilyroot for Marina and her father. Along the way, Handy and Marina fall in love, which makes it a tad complicated since they're from very different worlds.
Thank you to Char for letting me know about this episode!