I have often thought there is a species of human that lives in the waters. I have a good reason to believe this as well.
I had a congenital defect as a child and it was pronounced when I was a teenager....the usual age this particular congenital defect presents itself.
It's called a Branchial cleft cyst
How is started for me was a lump in my neck on the right side. Then it got so big it started to press on the artery going to my brain and started to give me headaches. My mother took me to a doctor that worked with my father in Chicago and he knew right away what it was.
He first put this huge needle in my neck a drained the fluid out. It took two of those big needles to drain it. Then he said it would require some surgery to repair.
When asked what it was he explained exactly what it was:
It was most basically from the embryonic stage we have what are exactly like gills... that function just like a fish. During that stage they close up at a certain point. Sometimes they do not completely dissolve and close. That is what happened to me. It only closed on the outer skin but remained open inside my neck.
So they did the surgery and closed the area up.
I'm very convinced that during our human development that there is a species in the water somewhere that is of a human evolution and developed with this kind of congenital effect that I experienced. I see no reason they did not develop. In that development they also most likely developed differently than we did outside the water with fins and fish like appearance with some human like attributes as well.
Here is what I was describing...and I will quote the part that explains that it is indeed from a gill like development during the embryonic stage that does not complete it's dissolvement.
The pharyngeal arches as seen during embryonic development
Branchial cleft cysts are remnants of embryonic development and result from a failure of obliteration of one of the branchial clefts, which are homologous to the structures in fish that develop into gills.
Branchial cleft cysts are congenital epithelial cysts, which arise on the lateral part of the neck from a failure of obliteration of the second branchial cleft in embryonic development. 
Phylogenetically, the branchial apparatus is related to gill slits. In fish and amphibians, these structures are responsible for the development of the gills, hence the name branchial (branchia is Greek for gills)