The mermaids in the title suggested to me a book that would be fanciful and pleasant. This was not the case. And frankly, even flesh-eating cannibal mermaids would have been more cheerful than the horrible atrocity at the end of the book, rivaling a war crime torture scenario.
The supposed story arc of the book is about Prestor John, king of mythical species such as unicorns, etc. If the voyage succeeds, the mythical creatures will continue their existence in the New World. If it fails, they will vanish from Earth forever.
The peculiar main storyline is about a pregnant young girl who may be carrying the messiah ... or the antichrist ... or perhaps the normally-conceived child of a normal man.
Let me state that I am a huge fan of Ms Friesner. I was collecting her works - and none of them hinted at this sort of book. She is an excellent writer! In fact, if she wasn't so good, I would not have become so immersed in this story and so shocked.
If you are considering giving this book to a teen or younger child, or an innocent and happy person, please reconsider. Or read it yourself first. I've read gritty military and science fiction books containing terrible crimes before ... but I wish I had brain bleach for this one.