Subject: Would a 'mermaid' or 'merman' feasibly survive in the ocean?

We've all heard of "mermaid-ism" (the stories on the news of those who've been 
born with their feet malformed and thus they resemble mermen/mermaids and 
require costly surgery to fix. Assuming another "foul-up" happened and someone 
ended up with a working set of gills (say, something ripped and forced the 
embroyo to adapt to survive) in addition to lungs, they also had the "fins" 
for feet, and they were healthy physically in every other way (say, the "fin" 
begins at just below the crotch), would they survive (let's say after age 17 
or so) if they attempted to live their lives in the ocean? What would an all-
seafood diet do to the human condition, for example? Would it be possible to 
engineer this, possibly, so we'd have more space to live in given our current 
population?

Re: Would a 'mermaid' or 'merman' feasibly survive in the ocean?