For a few years, there have been some produced-for-the-web mermaid series, often inspired by the various H2O TV series. One of these, the 3Tails series, has now spawned a 60 minute movie which can be steamed on Netflix!
Basic plot: The protagonist mermaids face the threat of some mysterious (and not-so-mysterious?) enemies that want to use their mermaid secrets/DNA for nefarious purposes. Meanwhile, the mermaids get schooled on plastics pollution of the oceans.