The classic merfolk are creatures of simple form: Above the hips, they are human, but below they have the body of a fish. This anatomy seems typical enough, if you don't think about it, but it quickly becomes incoherent if you do
Humans, being tetrapods, have roundish bodies filled with organs, with only a band of bone and muscle surrounding the guts and the spine placed at the extreme rear. Fish, on the contrary, are filled to the brim with muscle, with the spine running right down the dead centre of the body. Swimming vertebrates, especially large, relatively fast ones like mermaids, need a complete spine, not one which hops around in the middle of its length. Furthermore, there are issues in connecting the tail muscles to the rest of the body, given that fish do not have pelves and humans don't have large muscle-blocks like a fish
Being essentially swimming humans in behaviour, these merfolk won't need extravagant bones for perfect swimming. But certain humanoid features, like being able to sit down, would be quite useful. Also, given their proximity to the vertebrate plan, I'd like to keep them within a tetrapodal anatomy: Try and complete the spine, rather than adding in exotic bones found in your lower digestive tracts
Given these criteria, how could I connect together the body of these mermaids?