Tail includes the fluke (flat lobes at the end) but starts before that. We know this because pelvic bones of cetaceans are still vestigial and can be found arou

Tail includes the fluke (flat lobes at the end) but starts before that. We know this because pelvic bones of cetaceans are still vestigial and can be found around the "second third" of the body or so. So we know where the legs would be located and after this is all tail. Tail muscles do the bigger part of the work.