I tried googling fish sex. Ended up with a wikipedia article about sex for fish. (ugh, btw..)
Also some dude named Jorma Vulli who fucked a burbot in the 70's. Go funland.
edit: what the fuck, he fucked a fish and then cooked and ate it. And made a magazine article about it.
That was disturbing.
Anyway, fishes don't have sex. The male just spreads semen on the eggs eventually, but it doesn't involve any kind of penetration. There're probably some exceptions though.
Mammals, like dolphins, on the other hand, have sex. I found some articles about a man, and a woman, who had sex with dolphins. I'm not an advocate for zoophilia, because consent is dubious, but are dolphins intelligent enough to give consent, that is another debate.
So, we will go with the idea that mermaids are mammals (after all, they have mammaries), like dolphins.
But they can't go on scissoring! Sad...