Holland Oats wrote:
I am unaware of this theory.
I'm not sure how far you've got into the books yet but...
Arya eventually falls in with the Faceless Men. You may have heard other characters mention them as the ultimate assassins. They follow a death cult which holds that Death is the only true God because He appears in all religions ("there is only one God and His name is...") and believe themselves to be doing the work of this God when they assassinate people. They can also change their faces, as Arya finds out at the end of Clash of Kings.
This is why Arya's story arc takes her to practically every where in Westeros and has her constantly changing her identity. It was all set up for becoming a Faceless Man.
So, the idea is that Syrio Forel was a Faceless Man. He didn't die, instead he assumed the guise of one of the guards and eventually of Jaquen. He might even be the guy who trains Arya later on.
It's hard to tell how many Faceless Men there are because they change their faces so often.
Not sure whether I believe it but it is a fun theory.