The text plays on the alteration of the common saying 'If wishes were horses, beggars would ride,' substituting 'fishes' for 'horses' and 'cast nets' for 'ride.' While it does involve a change in the words, it does not create a double meaning or rely on similar-sounding words, but rather maintains the structure of the original saying in a whimsical way.
If wishes were fishes , we ' d all cast nets .