It was actually meant to remove impediments to navigation (the weirs blocked shipping lanes), but has since been interpreted as a removal of the King's title over the fishery (making the government a trustee over the fishery, not the owner). The public right to fish has now become rooted in common law.
As for being hung over taking small game, the Magna Carta also curtailed the King's "evil customs" relating to forests and warrens. These "evil customs" were to be "investigated" and "abolished completely and irrevocably". These, of course, were also meant to protect the landowner's rights as the King had a tendency to extend the boundaries of the "forests" at the detriment of the landowner's rights. It should also be noted that the charter was an agreement between the King and the Barons (not the commoners), however, since the development of our constitutional monarchy has made "all men equal," the rights reserved for the Barons are since extended to the commoners.
did it become common law after the fact?Navigation may have been the issue on larger rivers, but not all rivers.
the barons etc. still controlled law and punishment well into the 16th century employed as pseudo judges. This can be argued every which way.
I think that despite people wanting this to be a fair to all type event, it was simply a bunch of hereditary land owners trying to protect themselves and their from retribution from a King should the disagree with him or find themselves on his wrong side.
They could care less about the rights of his vassals or the common folk, not to be confused with those who lived on the common.
strange how a contract such as this, has been used as the basis of democracy and human right charters for close to a thousand years. Did Manx law adopt those principles?