I want to do it someday. Would be good fun. Maybe this summer. You would have to go with a couple of people though. Don't want to go drifting alone.
The one thing I'd be bothered with is sharks. I know its lame and pathetic, but you won't even see it coming.
The river would probably be safer

, because when the seabreeze turns off at night and goes to easterly. You won't be screwed as you would in the ocean.