You prob'ly have it right, Bill
Or at least close. The law says you must be at least 3 miles away from the nearest point of land (including any islands) on the US coastline. If you've drawn a line 3 miles from the Keys in any direction, that should put you where you need to be. The confusion comes where there are large bays or sounds that are more than 6 miles across, and people mistakenly believe they only need to be 3 miles from the nearest shore, even though they're still in the bay or sound. But that doesn't work. You must completely clear the bay and go 3 MORE miles out to sea in order to legally flush directly overboard or dump tank.However, there is something you do need to check: It's only on the Gulf side of FL, not the east coast (Atlantic side)...and I don't know whether the Keys are affected or not....but in some places the "3 mile limit" has been extended to 6, and in some places 12 miles. I THINK it's only further north in the bend around to the Panhandle, 'cuz 3 miles in one direction was inside 3 miles in the other...so they just moved the limits far enough out to keep that from happening. But otoh, there's been a lot of flap recently about the ecologicallly sensitive waters off the Keys, so it would be wise to make sure you're still ok at 3 miles.