Marina

My impression from what I've seen online is that the TV show Marina was a groundbreaking entry in the Filipino TV environ as the first fantasy teleserye. A fantaserye, as it were. (The Dyesebel movies already existed, but the TV adapations would only come after Marina.) Marina was hugely popular and was a trailblazer that paved the way for more Filipino fantasy shows, and you can see its influence in the mermaid shows that followed it.

The titular Marina (Claudine Barretto) isn't born a mermaid, but was cursed to transform into one when she was seven years old by the (human) witch Victoria, because of drama between Victoria and Marina's parents, Esther and Elias. Esther makes the difficult decision to give Marina to the sea, and she is raised by mermaid Queen Istah. As an adult, Marina gains the magical means to return to the human world, but Marina's mermaid transformation curse persists, while Victoria and her daughter Luna (who is also Marina's half-sister) are right there to give her new kinds of trouble.

It's very easy to see why the show did so well, with a large ensemble cast (human and magical) surrounding Marina that enables various relationships, problems and adventures, which then allow the story to go in all sorts of directions over its 188 episodes.

The mermaid stuff is pretty impressive for an early 2000s tv production, featuring multiple named mermaid characters with different tail designs and colours (amazing!), plus we get to see mermaid society and various kinds of mermaid and sea magic. The next most important mermaids after Marina are Lorelei (Eugene Domingo) who has her own antagonistic history with Victoria, and Sheila (Agot Isidro) who is Marina's best friend, but there are other supporting characters as well. The show has plot-driven underwater scenes sprinkled throughout, which look pretty good for tv.

The whole series is available to watch on youtube (but no English subtitles) :  Marina playlist