I'm trying to help my wife find the name of a book, possibly a book series. In this series, some people have the ability to transfer their consciousness into people on other planets that are too far apart for space travel to be practical. The two main characters are spies that fall in love and marry and have a descendant or daughter (possibly named Melody).
The folks that can do the transfer can only transfer a few times (up to 5 with 5 being very rare) because they then lose their sense of self.
She says that one of the worlds was a world of intelligent fish.