Actually, there was a very interesting Animal planet episode (http://www.youtube.com/watch?v=Cm3XdC-p8_I) on the recent news about the probable discovery of a mermaid body. In the show, scientists suggest that may be our evolution has had some roots under water. First, our body structure is far more plastic and ergonomic than the structure of a monkey. We are great swimmers in our movements and as you mentioned - our ability to hold our breaths under water for a very long time - the record is about 20 minutes! We can easily control our breathing which is something usually connected with marine mammals. In difference of other animals, we are almost hairless. Just like other marine mammals, we are born with insulating fat distributed throughout our bodies. This keeps us warm in water. All this takes place in the "Aquatic ape hypothesis". This view claims that our alleged ancestors went through a watery stage in our evolutionary development.
Of course, although all those facts connect very conveniently, the theory has not been proven yet by experiments and is so far just an idea that should be considered and investigated more.
In addition, some people still have marks from gills as an atavism.