So it's late in the afternoon and we are in the middle of the commerial Miami harbor. HUGE transport ships surround us and we are headed to the channel that supports the giant cruise ships. Our destination is just past and our marina is located across from the US Coast Guard station. We're cruising alone in this large channel - probably should have noticed there was something unusual about that? Ahead of us was one cruise ship at port, a police boat with blue lights flashing and a manned (and armed) coast guard ship eqipped with a 50 calibre gun mounted and at the ready. Our radio was on channel 9, the bridge contact, because we had gone through a series of bridges, some of which we had to have open for us. The standing channel for communication is 16. As Jim proceeded slowly a few of the Guardsman seemed to be waiving us back. But Jim thought they were waiving at the police boat. Then the police boat changed direction and started toward us. We thought to switch to 16 and a stern female voice advised that Homeland Security had closed this channel and we were to leave immediately. The guy standing at the 50 calibre was watching us. Although we were incensed to have to change our plans and truly believed the USCG should have posted some kind of warning at the channel entrance if they didn't want us there, we elected to be good citizens and do as we were told. Our exit from the channel was quite a bit faster than our entry had been....