I read a book, a really really REALLY long time ago (like when i was ten or something) called 'Mermaid's Tale' or something like that, and there was a scene in it where a mermaid had sex with a dolphin (I'd sneaked into the adult fiction section of the library.) according to my fuzzy memory of that book, since a mermaid has a horizontal tail like a dolphin,s not a vertical one like a fish, the internal organs would be more akin to that of a dolphin. There was even a mention of scaled labia flaps covering the slit, that would sort of retract or withdraw or open or something like that when the mermaid became sexually aroused.
And mermen were equipped like male dolphins.
I've never been able to find that book again. Wish I had at least made a note oe the author's name.