This is an old fingerplay that has many versions. I usually do the simplest.
Baby fish, do do, do do, do-do 4x open/shut thumbs & forefingers
Mama/Mommy fish, do do, do do, do-do 4x open/shut each hand
Daddy fish, do do, do do, do-do 4x open/shut both hands joined at wrist
Giant whale, do do, do do, do-do 4x open/shut whole arms