Besides territorial waters you also have zones farther out where a nation still has the authority to control things - the largest zone being the "Exclusive Economic Zone" which extends 200 miles from shore. This is set by the international "Law Of The Sea" treaty of 1982. Thus the U.S. government has authority to issue drilling permits for wells far out into the Gulf Of Mexico.
Of all the people I've seen talking about the spill, I am most impressed with Coast Guard Commandant Admiral Thad Allen. He understands the engineering challenges involved with working at 5000 feet below sea level. Rather than have the government step in and take over (they have zero experience in drilling oil wells), I'd much prefer to see expertise from other oil companies brought in to help BP. They may already be doing that.