There is a reason I specifically said "Eight Miles." You are in International Waters and the Coast Guard has no authority to arrest you if you are not putting lives in danger such as pirating other boats.
Many moons ago, we used to go out for multiple day fishing trips and use our AR-15's to our hearts content, shooting at "stuff" in the water. We scanned the horizon with our Bino's to make sure no marine or human life was within the range of our rifles. The thing you must do when doing this is to keep one person busy spotting for other boats and the occasional dolphin or sea turtle that will swim by. Safety must be exercised and good judgement practiced. Still, it is a great place to shoot.