This is going to be a tough one. I apologize in advance for knowing no more of this movie than a TV trailer.
Some time between 2010 and 2016, I saw an announcement for a creature horror movie involving mermaids on German TV. The film was going to run a few days afterward, hence it was not entirely new at that time. However, from the visual quality of the film, I also don't think the film could have been much older than from the 2000s. (Quite certainly not among the 1980s horror movies occasionally shown on that channel.)
Also, I am pretty sure it was not an originally German movie. (The trailer being shown on a TV channel as an announcement of an upcoming film shown on said channel, this probably means the trailer I saw was not necessarily an "official" trailer for the film, but might just as well have been a trailer assembled by the TV station to advertise the show.)
It is quite likely the channel I saw this announcement on was Tele 5, a channel that has somewhat specialized on showing "B movies"/direct-to-DVD productions/somehwat older and/or cheapish movies, in particular, regularly creature horror movies that fall into one of those aforementioned categories.
The one scene that stayed in my memory was the closing scene of the trailer: The scene starts with a camera pan (I think downwards into some kind of a vertical chute, but I may be mistaken about the exact presentation), at the bottom of which there is a mermaid. The background music rises gradually in pitch in such a way that it introduces a scream of the mermaid. Said scream beings sounding like a woman's voice (in pain/anguish?), but then transforms into a very unhuman-sounding voice1.
As far as I remember, the mermaid appeared to have at least a superficially human upper body in the trailer. That is, it looked more or less like a human woman (not sure about minor things like teeth or eyes), not like a remotely human-like monster. I cannot completely rule out the trailer gave a wrong impression in that regard, though.
What I think it's not:
1: The memory of which still manages to send chills down my spine, but I presume that is too subjective an impression to help in identifying this movie.