Re: mermaid porn:
It's really rare, actually. There's a decent amount of written work and occasional stills, but I've only ever seen 1 video of it, and it was just a mermaid masturbating. Apparently there isn't a desire to produce a video of a dude having sex with a woman with a fish tail.
Also, a common thing is to title/tag various porn vids with "mermaid", but it's just sex underwater. Girl w/ legs != mermaid.