Some countries like New Zealand and Australian have these - called Marine Reserves and limit commercial activities. They normally are in areas of great diversity (Milford Sounds) or international importance (great barrier reef).
or, like here in Perth, just off the coast of a major city.
There's a lot of local controversy because people believe that sharks living in the protected reserves are a danger to the people swimming off the city's beaches.
But politically this sort of conservation is very fragile. It's fine as long as no-one with any power wants to do anything profitable in the protected areas. But previous governments have caved in easily under commercial pressure.
