The narrator says he doesn't care whether you believe his claim that it's a mermaid or not. Good, because I don't. I think it's a hoax.
I'm convinced it's not a mermaid, because a mermaid is not something that fits into the evolutionary tree anywhere. Branches of that tree don't merge back together with branches from another limb, so a species that's half fish (or even half porpoise) and half primate just doesn't happen. My guess is that we've got footage of someone swimming in a costume tail composited with generic underwater footage, but I'll leave the details of how it was done to people with more expertise in video effects.
I invite all the creationists out there to go to the Great Barrier Reef to try to collect more evidence, though, as the discovery of a mermaid would be a big win for them, since evolution couldn't explain it.
Update: Does anything from the following movie clip look familiar?