I think a lot of my irritation would be because of the 'OMG COVERUP' folks. If something presents itself as a clear fictional documentary, that's fun, but when people are taking it seriously and I have to put on my 'Doctor Killjoy' hat to talk them down (or at least provide an opposing viewpoint for the bystanders), that is not fun at all. Basically 'how many people on the Internet am I going to have to argue with'?

I agree with you: ignoring the coverup to tell a documentary story about what scientists would do if they did find mermaids and wanted to study a non-human aquatic civilization would be a lot more entertaining to me*.

* Heck, one of my professors compared giving a research talk to telling a story in that you have a sort of narrative structure to work around and need to set up all your twists and turns and big reveals and such.